Question Problems after updating to 5.2

The 404 error only appears when trying to write a "quick reply".
When trying to post a new topic or a regular answer, you just end up in the list of topics and nothing happened - no answer or new topic was posted.
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Disabling the "add file ending to URLs" option in the Joomla configuration didn't help.
The "URL rewriting" option is also used, but I cannot just turn this off - it is required for the website. The old version of the forum didn't have any problems with these settings, by the way.
Switching to the Protostar template and then trying to post a new topic did not display any message at the top.

In your configuration report I see, the captcha is enabled for guests and registered users. CAPTCHA Challenge for Users = 3. That's why I could not send a message, because it lacks the math field.
Maybe I had right, as I suspected after the first login in your page, that the captcha causes the problem. Random has a user reported today a similar problem (see here) .

Hi,
no - I'm sorry, that's not it...
I have set the Kunena "display captcha" option to "nobody", the field "captcha challenge for users" is empty (was set to "0" before).
In the main Joomla config, "standard captcha" is set to "no selection".
The captcha plugins "recaptcha" and "invisible recaptcha" are both deactivated. As per the linked thread I also deactivated "easycalcplus" plugin.
Then emptied cache, tried again, but the exact same things happen (logged in as admin):
"New topic" or "Actions - Answer" displays the list of topics, but the new topic or answer have not been posted.
"Quick reply" leads to a 404 message.
There are no pending messages on the index page of the forum.

I'm at a loss. Is it possible, that menu items with other user access are present? Check all menu items (also disabled and trashed items), all that can cause conflicts.