We had problems with xcache in the past, but only on the front end, (gallery tab disappearing at some moments) but there shouldnt be any problems on the back end..
Do the following.. disable vboptimize and see if the error persists..
If yes, there is a workaround that will fix the front end..
somehow... I dont know why, x-cache, sometimes sets the vbgallery datastore to empty (allthough it is fetched like all the rest)
it is then regenerated at the moment when you save any options..
hard coding those 2 options will probably work on the backend..(as they are the only 2 needed) but certainly not on the front-end..
There is a very clumsy workaround for the front end.. (it adds a query, but only on the moment where the settings are gone.)
I will install xcache on my localhost and see if i can reproduce the problem.
For the moment do this:
1. check if vboptimize is the issue.
Disable the product, and see if it happens again..
do the following:
$ppg_vars = array();
and right below add:
Content visible to verified customers only.
Keep the start and end rems, so you can remove it when a definite fix is found.
Let me know if this removes the front-end error.