In the 'Find' dialog, there's an option 'Ignore search settings of groups'. Thank you, I didn't know this option. That's exactly what I was looking for.
Is it expected behavior, that for searching in a folder via right click > "Search in this folder" the "Search entries in this group" property of the group needs to be activated? Well, of cource technically it could be considered as correct behavior, not to find anything in a group, if the search for this group is disabled. But it's kind of confusing, that an explicit search with "Search in this folder" doesn't overrule this setting. Or at least warns the user, that he's trying to search in a group...