Security error installing particular apps

Someone at 34c3 showed me this problem he had installing one of my apps from FDroid:

He can install other apps of mine - but not this one. Not really sure what I can do about this - any hints on what I can do to help him are welcome.

That’s a system prompt, that doesn’t come from F-Droid. If F-Droid can already install some apps but not others, then something odd is going on. Or, the user has two copies of F-Droid install (e.g. official and nightly build) and only one is allowed to install.

Thanks - will ask the user when I see him again if this is the case here.