Or simply the groups info failing to load for that session... It may happen, even though it's a rare occurrence. In this case you could find yourself with an empty or partially loaded groups list and, possibly, with group info failing to load. The cure is to "Clear Group Cache" (in the Advanced menu), or even more radical, to open the Group Titles floater (right click on your Av and choose "Titles...", or use "Edit" --> " Group Titles...") and hit the Refresh button of the floater; if this still fails, a relog is in order.
So definitely not an UI issue. The dark skin doesn't use modified menu definitions, only modified colors and icons (the Silver skin is the only one using a few modified menus). Also, since you are using the English UI, there is no risk of a corrupted/faulty translation XML file being used and possibly crippling the group info panel.
The log file is not only useful for crashes: it contains a load of info that is relevant to what happens during a particular session, and warnings issued there can often explain what went wrong at a particular point (not all warnings are relevant or even the sign of an actual issue, but when something bad happens, there's often a relevant warning).