OS X is repeatedly asking for login keychain password

I had exactly the same problem with Lync messenger - request to enter a keychain password, over and over again, even though password was the correct one. Tried numerous suggestions, nothing worked. Until I found this one:

• go to the folder userid/Library/Keychains
• look for a key with the file name from the password request window
• delete this file (only this one!)
• try again your app - password is accepted now

Somehow this key was not shown in Keychain Access, like it does not exist. But actual key file was present in keychain folder.

Seems to be some bug in macOS (I am on Sierra) when it can't see key-file, and then another bug - system tries to create a new key-file, but file operation fails (file already exists there). I would suggest to the apple devs to add a proper exception handling there, but I am satisfied for now - problem fixed.

I just disabled FileVault under my System Settings > Security & Privacy last night and let the HD decrypt again and voila, now the keychain works fine again. I had activated it only recently, but had no idea that this would cause such problems and side-effects.

(Other issues with FileVault are probably related, such as Reset Password app in recovery mode not showing and disks)

A boot into Recovery Mode (Command + R right after reboot) and Disk Utility > First Aid on the main disk solved the problem for me.