I tried now the tagging-software "The Godfather".
This software does not change the displaying behaviour in FrodoPlayer like Tag&Rename does.
So it seems to me that:
1) CDex files seem fine
2) The Godfather ogg-files seem fine
3) Tag&Rename ogg-files are NOT fine in FrodoPlayer but in WinAmp
4) re-tagging files that were once changed with Tag&Rename with "TheGodfather" does not repair them for me.
I re-installed Frodoplayer several times and alway rebuild the database.
THX for any help in getting my ogg-files to run again in advance.