Login fails on Confluence

Hi,
A few users sometimes get an error when trying to log in to the confluence host hsv5045s.vdad1.bcv.ch:8090
However their accesses are correct, and sometimes the login works during a certain period.
There seems to be a way to "reallow" the login by some kind of synchro done by admin users but it shouldn't happen so often.
Also there seems to be no precise error, just a text saying wrong credentials.
If you need specific logs or server infos try to contact our server admin, by sending a mail to fbus@ch.ibm.com
Thank you