Firefox - flashing login screen

Since very recently I am having the following problem with some sites:

The input fields are correctly filled, however with reloads (“flashing”) the login window seems to be reloaded again and again with the credentials filled-in. It is not possible to execute the login.

I am experiencing this with the following sites

Windows 10 / Firefox 86.0 / KeePass 2.47

Do you have auto-submit enabled? If so, disable it, at least for the sites that have this problem.

Make sure you’ve checked out the troubleshooting guide too.

No, auto-submit is not enabled. I checked the hints in the troubleshooting guide, also ran the debugger - all without success.

I just performed a test on Chrome, there I have the same symptoms.

I have solved this for me with moving to KeePassXC, where I don’t experience this problem.