We've been dealing with this as well for a while. It's only affecting a handful of users and we too have been unable to reproduce it. Support says it's a client side issue and has been working with us to troubleshoot. There's a KB article that deals with time sync being the cause, so give that a try:

This article didn't help us solve it, but maybe it will for you. 

I have seen some other threads recently on this topic as well so we're not alone. I would suggest opening a support ticket so they can take a look. With more data on the issue they might be able to stumble on a solution for us all.