I tested the Flitsmeister-App using apkpure and also can't install it on my Jolla C. But it's the first APK that doesn't work - all my other Android apps installed right away.
Then I try to install your app on Android Cyanogenmod 13 (Google free one) - also didn't work!

About 2 years ago I had a regional messaging app installed on Jolla1. Surprisingly I couldn't install the same app on Cyanogenmod (same error you have now). When I logged the install process, I found out that installation failed because of missing libraries (in that case Google map APIs). After manually adding them, installation worked.

But I never figured out why I could install it on Jolla1, without Google Play or anything googlish installed, but not on Cyanogenmod Android. I'm no pro in such things at all, but looks like Alien Dalvik on Jolla 1 had something set up to bypass this issue.