Error 404 when creating a manual post on postcast page

Describe the bug

I installed Castopod 1.13.3 on https://wuepod.de/@wuepod. (Almost) Everything works fine, I logged in, successfully imported my Podcast, changed settings, edited episodes, and so on. But when creating a manual post on the podcast page logged in as admin: when I click the send-button, the URL https://wuepod.de/@wuepod/post/new results in a 404 error. And no post is send to the fediverse.

Steps to reproduce (at least for me)

1. Install Castopod on a server (using Uberspace as the provider).
2. Change .htaccess for Uberspace limitations: Options -Indexes (without the "All"), Options +SymLinksIfOwnerMatch (instead of "Options +FollowSymlinks") and RewriteBase /
3. Log in as admin
4. Create a manual post on the podcast page on the frontend.

Expected behavior

The post should be posted to the fediverse followers and no 404 should show up.

Actual behavior

The browser displays a 404 error when sending the post from the podcast page.

Relevant logs and/or screenshots

If this helps: When I add

error_log("Request URI: " . $_SERVER['REQUEST_URI']);
error_log("Query String: " . $_SERVER['QUERY_STRING']);

to the index.php, this is the log output

[01-Feb-2025 00:35:19] WARNING: [pool www] child 12105 said into stderr: "[01-Feb-2025 00:35:19 Europe/Berlin] Request URI: /@wuepod/posts/new"
[01-Feb-2025 00:35:19] WARNING: [pool www] child 12105 said into stderr: "[01-Feb-2025 00:35:19 Europe/Berlin] Query String: /@wuepod/posts/new"

Context

• Castopod 1.13.3 from the Castopod Package
• Linux CentOS 7 (Uberspace.de)
• Browser: Firefox/Vivaldi
• Webserver: Apache