This is the weirdest bug. If I enable certain widgets on a widget page then this will display a white page with the message: The board is currently being upgraded. Please check back later. If the widget page is an index page then the whole site will be broken. I have tried to narrow it down to certain widgets and have found: - If I enable a custom XFMG widget it happens - If I enable Nobita Groups widget is happens - If I enable XFRM latest resources it happens in 50% of page loads. This is really weird.
DB and LiteSpeed web server. But not the litespeed cache addon installed on this particular XF install yet.
Are you still having problem with the latest 2.6? I think this might happen because of and issue with the cache system (memcache or similar, not litespeed cache). Please try different caching storage option for widgets and see whether the problem go away.