Hi,
I'm thinking that this is related to your upload media issue and the error message on the Media and Quota tab... all 3 things are linked to server sessions which maintain your login across the different windows and during upload (for security).
Personally I've not seen the issue on any of my installs, although as Julian says, he has and one or two others have reported similar intermittent issues, but as yet we haven't been able to recognise any common ground, server settings or anything that would cause this. It must be a server setting or something that is not being picked up or blocked by the installer as there are hundreds of installs all working without reporting this problem...
Therefore the more info we have about both your systems the more chance we will have of solving this, mainly, but not exclusive, these things would be a useful start:
MySQL version (if known)
PHP version (if known)
Browsers tried
Version of XOT (found at /version.txt)
php.ini settings
It would also be useful if when you experience it again, if you could logout, clear your browser's cache, refresh the page and login again and see if the error persists.
Thank you for your patience while we try to debug this annoying issue.
Regards
John