Hi! I have the same problem as juuso. Got that just after latest update (i think yesterday but not sure).
message from the log is:
WARNING [java_downloader] -> The Plugin for fileserve.com seems to be out of date(rev13069). Please inform the Support-team
http://jdownloader.org/support.
(sorry, I don't want to paste whole log because I have no idea what data is confidential there)