Unable to use webmail (Internal Server Error 500)

[LOGODERKING]

If I try to use the Browser Webmail I keep getting the following error code:

Failed to load resource: the server responded with a status of 500 ()

webmail/#?action=com…b8233cd8bf4f03b2e:1 Uncaught SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON
    at JSON.parse (<anonymous>)
    at Function.parseJSON (jquery.min.js:3:4322)
    at Webmail.<anonymous> (webmail.js:1063:26)
    at Object.i (jquery.min.js:3:6336)
    at c (jquery.min.js:3:7857)
    at Object.fireWith [as rejectWith] (jquery.min.js:3:8658)
    at k (jquery.min.js:5:14040)
    at XMLHttpRequest.r (jquery.min.js:5:18226)
    ```
But the thing is if I eg. try to use a programm nextcloud to send the mail over the smtp port it works without any problem, so the problem is something with the Webmailer. Also the webmailer worked yesterday without any problems but it just suddenly stopped working. The System runs on Debian 12 and I used the official install programm.