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.