If a custom group for an entity that isn't a contact includes anything in the extends_entity_column_value, then Drupal Views is broken (e.g. you get an error on the views admin page).
This appears to have been triggered by some additional security bits in the latest releases, and is particularly obvious with civivolunteer installed. I don't know why civivolunteer is including anything in that column and/or whether it should, but the important bit is that the views code seems to assume that that field is a contact subtype.
I'll attach a patch that is working for me.