First column icon Meaning what does "white cross on red circle" mean?

[Bruno1]

The first column on the main display corresponds to the file type.
I have a lot of MP3 files that

• work properly
• but display this icon
  
  instead of the normal MP3 icon, , which I get for the most MP3 files
  why?

[ohrenkino]

See the changelog

[2024-07-18] CHG: MP4 files that couldn't be parsed are now listed in the File List with an error description in the Tag column. (#65311)
This also applies to other file type for which errors have been detected in the tag.

See also here:

MP4 File Checker Questions & Suggestions

In version 3.27 I have been testing the new white cross error marker for MP4 files with good results so far. None of my M4a files had problems and only three MP4 files were marked. But: Am I correct in assuming that M4a files are checked, since those use MP4 containers? Am I correct in assuming that AAC files are not checked, even though they were extracted from MP4 containers? As for the messages, I have seen only "ERROR cannot parse...". Are there other messages? If there are, please li…