We've been working with MultiTerm Administrator for over a year but this morning when I tried to login I got the error message

Value cannot be null.

Parameter name: usrCredentials

Curiously, that error message doesn't always appear – sometimes I get this:

I came across this Knowledgebase article and did as it suggests, but I still can't login, though the problem then changed from the above message to this:

Can anyone suggest what might be causing the problem and/or a workaround?

And why might this have started recently? I don't think there have been any updates to MultiTerm Administrator.

Also, that Knowledgebase article seems to date from 2014 and says "The SDL Development team is aware of this problem and is working on a solution" and yet here we are with the problem in 2017.