Option to switch back to the old UI?

[stphn]

What server / mail provider are you using? Might be that it changed to different authentication schema which IMO also wasn’t supported by 5.600

E.g:

How to set up with a Gmail account

Gmail supports IMAP. Note that Google also prefers a more secure authentication protocol called XOAuth 2.0 which is currently not supported by K-9. To configure access to Gmail in K-9, just follow the steps below: Enable IMAP in your Gmail settings. If you have 2FA enabled for your Google account, visit https://myaccount.google.com/security#signin. Click on “App Passwords” and generate a new password. This is the password you will use for IMAP, instead of your Gmail password. Open K-9 and …

In the meantime I think Gmail requires OAuth2