XF 1.5 User been logout from forum

[Sunka]

One user send me a message that sometimes he has been logout form forum, while he try to post.
He send me a screenshot where is visible that he is loged in forum, and error message (when he click to post a message/post) that he must login if he wants to post a message.
Any thoughts about this?
Browser cache, some bug...?

 

[Mike]

If their IP changes and they haven't selected stay logged in, that can happen (as their session becomes invalidated). In general, staying logged in can workaround these issues. If it doesn't, that would either indicate that their browser isn't consistently sending cookies (tends to be unlikely) or your server isn't getting/seeing the cookies (which we have seen with caching proxies).