Thanks Rejetto,
In fact, I often restart my development server when updating my template, and only rarely get this error message. So, not directly caused by a server restart. Perhaps a server restart in conjunction with a previous cookie expiring?
Also, once it does occur, it persists through browser restarts, page reloads, etc. That is, if I restart the browser, then when I go back to my server, I have to log in again (as expected), and after login, Renames will still fail with the same error about a bad session.
Not sure if a server restart after the error occurs will clear the situation. I don't think so, but not 100% sure I've tried this explicitly.
Clearing cache in the browser doesn't help, with or without a browser restart; this was explicitly tested.
Clearing cookies in the browser does resolve the issue.
(Again, I don't have reports of it happening in the field, though I warn users of the possibility in my instructions. It may be just a side effect of me running sites with the same template on two different servers, one at localhost on my dev machine, and one online at a global static IP, with the same logins on both, and my browsing back and forth between the two sites. The actual users use their single production site only, not two different ones.)
Locally, I've had it happen sometimes with both Firefox and Chrome, on Win7 and Win8 clients, and with a Win7 and Win8 server. Also, it happens sometimes to the localhost server, and sometimes to the remote server. But, I've not found the pattern that will reproduce it.
I'll keep trying to get a reproducible error, as I don't think we can hope to fix it without that. I'm seeking background on the function of cookies to guide me in finding a reproducible cause.
Cheers,
Steve