Hello Jannis
Thanks for posting in the Community!
We support the following web browsers:
As it looks like your Firefox browser is up-to-date I would recommend as a first step that you try in a Firefox incognito or private window.
Could you also try connecting to a different network or disconnecting from your VPN (if applicable)? Sometimes account performance issues can be related to internet security/speed or browser issues.
These steps will help us eliminate that possibility. For help with opening an incognito window please see this community post (in English).
Lastly if you do not see any difference when trying the steps above can you please ensure that third-party cookies are enabled in Firefox? The following article explains how to enable third-party cookies in Firefox: Enable third-party cookies
Please let me know in case you still encounter the issue you described after having followed these 3 steps!