diff --git a/php/public/index.php b/php/public/index.php
index 9d34eabd..f2ee97c9 100644
--- a/php/public/index.php
+++ b/php/public/index.php
@@ -112,7 +112,13 @@ $app->post('/api/configuration', \AIO\Controller\ConfigurationController::class
 $app->post('/api/desec/register', \AIO\Controller\DesecController::class . ':Register');
 
 // Views
-$app->get('/containers', function (Request $request, Response $response, array $args) use ($container) {
+// Shared closure that renders the containers page.
+// $desecVars can override 'desec_show_password', 'desec_prefill_email', and 'desec_error'.
+$renderContainersPage = function (
+    Response $response,
+    Request  $request,
+    array    $desecVars = [],
+) use ($container): Response {
     $view = Twig::fromRequest($request);
     $view->addExtension(new \AIO\Twig\ClassExtension());
     /** @var \AIO\Data\ConfigurationManager $configurationManager */
@@ -130,7 +136,7 @@ $app->get('/containers', function (Request $request, Response $response, array $
     $bypass_container_update = isset($params['bypass_container_update']);
     $skip_domain_validation = isset($params['skip_domain_validation']);
 
-    return $view->render($response, 'containers.twig', [
+    return $view->render($response, 'containers.twig', array_merge([
         'domain' => $configurationManager->domain,
         'apache_port' => $configurationManager->apachePort,
         'borg_backup_host_location' => $configurationManager->borgBackupHostLocation,
@@ -185,8 +191,39 @@ $app->get('/containers', function (Request $request, Response $response, array $
         'desec_password' => $configurationManager->desecPassword,
         'is_desec_domain' => $configurationManager->isDesecDomain(),
         'desec_account_registered' => $configurationManager->isDesecAccountRegistered(),
-    ]);
+        'desec_show_password' => false,
+        'desec_prefill_email' => '',
+        'desec_error' => '',
+    ], $desecVars));
+};
+
+$app->get('/containers', function (Request $request, Response $response, array $args) use ($renderContainersPage): Response {
+    return $renderContainersPage($response, $request);
 })->setName('profile');
+
+$app->post('/containers', function (Request $request, Response $response, array $args) use ($container, $renderContainersPage): Response {
+    $email    = (string)($request->getParsedBody()['desec_email']    ?? '');
+    $slug     = (string)($request->getParsedBody()['desec_slug']     ?? '');
+    $password = (string)($request->getParsedBody()['desec_password'] ?? '');
+
+    /** @var \AIO\Desec\DesecManager $desecManager */
+    $desecManager = $container->get(\AIO\Desec\DesecManager::class);
+    try {
+        $desecManager->register($email, $slug, $password);
+        return $response->withStatus(303)->withHeader('Location', '/containers');
+    } catch (\AIO\Desec\AlreadyRegisteredException $ex) {
+        return $renderContainersPage($response, $request, [
+            'desec_show_password' => true,
+            'desec_prefill_email' => $email,
+            'desec_error'         => $ex->getMessage(),
+        ]);
+    } catch (\Exception $ex) {
+        return $renderContainersPage($response, $request, [
+            'desec_prefill_email' => $email,
+            'desec_error'         => $ex->getMessage(),
+        ]);
+    }
+});
 $app->get('/login', function (Request $request, Response $response, array $args) use ($container) {
     $view = Twig::fromRequest($request);
     /** @var \AIO\Docker\DockerActionManager $dockerActionManager */
diff --git a/php/src/Desec/AlreadyRegisteredException.php b/php/src/Desec/AlreadyRegisteredException.php
new file mode 100644
index 00000000..54ce2ee2
--- /dev/null
+++ b/php/src/Desec/AlreadyRegisteredException.php
@@ -0,0 +1,17 @@
+<?php
+declare(strict_types=1);
+
+namespace AIO\Desec;
+
+/**
+ * Thrown when the deSEC API reports that the supplied email address is already
+ * associated with an existing account (HTTP 400 with an "email" error field).
+ */
+class AlreadyRegisteredException extends \Exception {
+    public function __construct(string $email) {
+        parent::__construct(
+            'This email address is already registered at deSEC. '
+            . 'If this is your account, please enter your deSEC password in the password field and try again.',
+        );
+    }
+}
diff --git a/php/src/Desec/DesecManager.php b/php/src/Desec/DesecManager.php
index 13f7a08b..9ac45d41 100644
--- a/php/src/Desec/DesecManager.php
+++ b/php/src/Desec/DesecManager.php
@@ -122,10 +122,7 @@ class DesecManager {
         if ($code === 400) {
             $data = json_decode($body, true, 512, JSON_THROW_ON_ERROR);
             if (is_array($data) && isset($data['email'])) {
-                throw new \Exception(
-                    'This email address is already registered at deSEC. '
-                    . 'If this is your account, please enter your deSEC password in the password field and try again.',
-                );
+                throw new AlreadyRegisteredException($email);
             }
             throw new \Exception('Registration at deSEC failed (HTTP 400): ' . $body);
         }
diff --git a/php/templates/includes/desec-register.twig b/php/templates/includes/desec-register.twig
index 6a015194..2b9e97f7 100644
--- a/php/templates/includes/desec-register.twig
+++ b/php/templates/includes/desec-register.twig
@@ -4,7 +4,8 @@
     {% if desec_account_registered %}
         <p>Your deSEC account (<strong>{{ desec_email }}</strong>) was registered successfully but the domain could not be registered. Please enter a desired subdomain slug (the part before <code>.dedyn.io</code>) and try again, or leave it blank for a random one.</p>
         <p>Your deSEC login credentials (for <a target="_blank" href="https://desec.io">desec.io</a>): Email: <strong>{{ desec_email }}</strong>. <details style="display:inline"><summary>Reveal deSEC password</summary><strong>{{ desec_password }}</strong></details>. Please save these in a safe place.</p>
-        <form method="POST" action="api/desec/register" class="xhr">
+        {% if desec_error %}<p class="error">{{ desec_error }}</p>{% endif %}
+        <form method="POST" action="/containers">
             <input type="hidden" name="{{csrf.keys.name}}" value="{{csrf.name}}">
             <input type="hidden" name="{{csrf.keys.value}}" value="{{csrf.value}}">
             <input type="text" name="desec_slug" placeholder="my-nextcloud (optional)" pattern="[a-z0-9]([a-z0-9\-]{0,61}[a-z0-9])?" title="Only lowercase letters, digits and hyphens (1–63 characters). No leading or trailing hyphen." />
@@ -12,12 +13,17 @@
         </form>
     {% else %}
         <p>Please enter your email address. You can also enter a desired subdomain slug (the part before <code>.dedyn.io</code>); leave it blank for a random one.</p>
-        <p>If you already have a deSEC account for this email address, enter your deSEC password in the optional password field below to log in with it instead of creating a new account.</p>
-        <form method="POST" action="api/desec/register" class="xhr">
+        {% if desec_show_password %}
+            <p>If you already have a deSEC account for this email address, enter your deSEC password in the optional password field below to log in with it instead of creating a new account.</p>
+        {% endif %}
+        {% if desec_error %}<p class="error">{{ desec_error }}</p>{% endif %}
+        <form method="POST" action="/containers">
             <input type="hidden" name="{{csrf.keys.name}}" value="{{csrf.name}}">
             <input type="hidden" name="{{csrf.keys.value}}" value="{{csrf.value}}">
-            <input type="email" name="desec_email" placeholder="your@email.com" required />
-            <input type="password" name="desec_password" placeholder="deSEC password (only if already registered)" autocomplete="current-password" />
+            <input type="email" name="desec_email" placeholder="your@email.com" value="{{ desec_prefill_email }}" required />
+            {% if desec_show_password %}
+                <input type="password" name="desec_password" placeholder="deSEC password" autocomplete="current-password" />
+            {% endif %}
             <input type="text" name="desec_slug" placeholder="my-nextcloud (optional)" pattern="[a-z0-9]([a-z0-9\-]{0,61}[a-z0-9])?" title="Only lowercase letters, digits and hyphens (1–63 characters). No leading or trailing hyphen." />
             <input type="submit" value="Register free domain via deSEC" />
         </form>