HTTP Web Server: Invalid POST Request Exception [/names.nsf?Login]

Recently Updated the server from 6.0.1 to 6.5.1From that update on I receive following error

HTTP Web Server: Invalid POST Request Exception [/names.nsf?Login]

Following funny thing occurs:

I cannot log onto certain web services, but if double click the send button (from the Log on Screen)it works…? Anyone who can help me?