Firefox Web Login not working

All of a sudden today I cannot log into the web interface on any of my duplicati instances via Firefox.

Still works fine in Chrome.

Get “Login Failed: Unauthorized”

I deleted cookies and it still doesn’t work.

Did you try Private Browsing mode in Firefox? (Private Browsing - Use Firefox without saving history | Firefox Help).

This has happened to me sometimes that some websites do not work even if I clear cache, and I try them in private browsing / incognito mode and it suddenly works.