MP3 Diags reports errors after inserting coverart

I have bought some MP3 files via Amazon and checked them with MP3 Diags. They were fine. Then I used the add coverart function in MPTagThat to add a front cover. After that MP3 Diags reports errors:

- ID3V2 tag has an APIC frame, but the image could not be loaded.
- unsupported text encoding for APIC frame in ID3V2 tag

Is that an error in MPTagThat or in MP3 Diags?

Reproducing the error I have noticed that I have made a mistake and gave a wrong description of the error. I just copied the files again from the original source and discovered that they already have covers in the folders as well as in the files itself. Now I add a song text and save the file with MPTagThat. After saving the file with MPTagThat it is reported as having errors by MP3 Diags:

- ID3V2 tag has an APIC frame, but the image could not be loaded.
- unsupported text encoding for APIC frame in ID3V2 tag

MPTagThat must change soming on the APIC Tag when saving the file again.

It's done. You should have a PM with the link and password.

Thanks for the effort and the quick response.
I thought it was better to save the text in unicode. So maybe it is just better to ignore the MP3 Diags messages or ask the author to look into the issue.