Quick solution: Disable "Read: APE" in the tag options.
In the caption of the Tags window you see: APEv2 (ID3v1 ID3v2.3 APEv2)
That means your MP3 has 3 different tag types on it: ID3v1 ID3v2.3 and APEv2
And the fact the APEv2 is also displayed before the brackets meands that Mp3tag currently shows you the APEv2 tag (and only this tag)
The cover gets only saved in the ID3v2 tag, but you are seeing the APEv2 tag, so no cover.
So if you disable "Read APE" you should see a cover.
You could also remove the APE tag as Detlev said, it won't give you a big advantage and it is not supported by a lot of programs anyway.
To remove only APE tags, you must select only APE in the tag options in the "Remove" part.
Then just click or press Ctrl+R