The reason of the problem is that a certain API command, for reading the account list, does not work anymore since last week. This does not affect running sessions, but it affects login to Oanda.
We've uploaded a modified Oanda plugin that does not use this command. You can download it here: http://opserver.de/down/Oanda.zip.
You can login with this plugin, but must now enter your account ID in the User field. Leaving the field open does not work anymore.
I hope that Oanda fixes the problem soon.