When inserting the member directory module on a page, I get an error displayed above the input/search field: 

"An error occurred while processing the request: error"

I just upgraded to DNN 7.3 but I haven't been using the module before, so I don't think this is related to the upgrade, i.e. it didn't break because of the upgrade.

I've seen this error mentioned in the forums and in other questions, but I can't solve it.

I have updated all records in UserProfile where propertyvalue was null, as suggested in another post. But it didn't help.

There are no entries about this error in the event log.

It works fine when changing the "Filter by" to "User", however when selecting any of the other options ("No filter", "Group" etc.) the error re-appears.

Is this a known issue? Could it be caused by a corrupt user profile of some kind?