Google Music 'Unknown Artist' only in Android App

So I have an album that for some reason shows up as of an 'Unknown artist' in my library in the Google Music app. I already fixed the tags in Foobar (and also checked them again with MP3Tag) and then deleted the old files off my sd card and copied the fixed ones back on it. Yet still, they show up without an artist in the app...

I tried several things:

• Uploading them to my Google Music Library, then they show up just fine. (So the files should be correct. But I want them on my extra SD card, so this was just to see if there was still something wrong with the files.)


• Clear all the data of the Google Music app (menu->apps->google play->clear data.) Didn't change a thing.


• Reinstall the Google Music app.


• Remove the com.google.android.music folder.


• Restart phone.

Nothing fixed it... Now I don't know what else I could try. Obviously something went wrong, since at first the tags were wrong, but I fixed them. Any other program I tried reads them just fine. Any ideas on how to fix it?

Edit:

I just tried another music app on android (doubleTwist) and that also shows those songs under 'Unknown artist'... Yet when I copy the file back from my SD card to my pc it does show both Artist and Album artist correctly. I'm stumped. Is there some kind of cash of android I can clear? (Already tried clearing the Google Music app cash.) I'm completely stumped on what's going wrong :(

Edit 2:

I just found that the 'unknown artists'-songs apparently use an older/different version of Flac than the other files. According to foobar the bad songs are flac 1.1.1, while the other files are 1.2.1. When looking in Mp3tag the bad ones says 'FLAC (FLAC ID3v2)' and the good ones say 'FLAC (FLAC)'.

Thanks a lot!