oops an error occured and session cannot be null

Please help me with a problem! I have files to share with some friends from ccloud webapps my account and from 3-4 months appear a new box with these messages : Oops an error occured… Session cannot be null and a blue box with OK. Only when it’s pressed it can be use the filles on the account. Thank you for your help!

http://tinypic.com/r/160pz6x/8   printscreen with the error