Xtemplate error after login?

Don't know if this is known already:
Xtemplate degrades (gracefully but not satisfactorily ) when I log in to the system. That is: It only works when not logged in.
When trying to change template I get errors: This required field is missing:
(think this has been reported)

It is a shame that I should keep two different browsers open in order to see my themed version of the site, while maintaining the content.
No big deal otherwise.