My gallery is http://www.peppermint-bay.com/cat/
I've had this problem for awhile now, but didn't feel like looking into it since the gallery works fine.
I just upgraded to 1.5.16 and the problem still exists.
When I login as admin and go to Information, Check Versions.. if I try to do full screen it waits for about 20 to 30 seconds and says
"500 - Internal Server Error".
Even if I uncheck that box that says to uncheck if I get error it still gives me the same problem.
Text is fine.. it's just I'd rather have full screen because it's easier to read.
So you get the HTTP status code 500 just at the version check?
Some information about that error: http://en.wikipedia.org/wiki/List_of_HTTP_status_codes#5xx_Server_Error
If everything else works as expected you shouldn't worry.