Thanks for trying to replicate. I found the cause.
Clearing the cache didn’t work, and deactivating the cache plug-in didn’t either. That meant painfully disabling each plug in and testing as you suggested.
It turns out the offending plug-in was “Google Analytics Dashboard for WP” by ExactMetrics. The problem disappeared when it was disabled. As it happens, it was one of several plug-ins that was updated at the same time as the GP theme and GP Premium plug-in.
I noticed another update was available from them already so updating this plug-in from ExactMetrics from 6.2.0 to 6.2.2 (current) fixed the problem.
None of the other GP users will likely encounter this problem, but posting the solution makes me feel better. Well, maybe a little. ๐