My iCloud accounts have disappeared from Mail
Yesterday everything was fine. Today, I went to reply to an email to discover that instead of my reply email being sent from my iCloud account my personal Yahoo email address was shown. I clicked to get the selection menu to see that all my iCloud accounts have disappeared! I've only been doing innocent user things, so it's not something I've caused.
When I look in Mail > Preferences > Accounts I see that the left window is too narrow to see the full name of each account, which is odd. All my icloud addresses have disappeared except for the one I normally use where there is an entry. "Enable this account" is ticked, the description in correct but the Email Address field is blank. I can't edit the feld.
I get a pop up window that I can't edit:
What has happened? I haven't installed any new software or anything.
[Image Edited by Moderator to Remove Personal Information]
MacBook Pro 15″, macOS 11.6