With the hints provided, I solved the issue.

Replied via ticket #2752 :

Hi, based on the debug log the file is stored in your library as
D:\Musik\Klassik\Cleveland Quartet\Schubert- Forellenquintett D 667\01 - Forellenquintett A-dur D.667 - 1. Allegro vivace.flac
but on the disk it appears to be in folder
D:\Musik\Klassik\Cleveland Quartet\Schubert - Forellenquintett D 667\

So I think that another app must rename the file (adding the blank space) and thus this file is deadlink in MM5 library and I suppose that you even cannot play it in MM5 and it appears under
Music > Files to Edit > Deadlinks node, right?

You can either adjust the path in MM5: right-click the file > Properties and change the path to match the path on the disc.

Or use "Locate moved/missing tracks feature" , see: https://www.mediamonkey.com/wiki/index.php/WebHelp:Adding_Existing_Files_to_the_Library/5.0#Manually_Locate_Moved/Missing_Files
for more info how to handle deadlinks.

Just submitted as requested.

Can you please capture this in a debug log (step 4b) and attach the log to a Support Ticket: https://www.mediamonkey.com/forum/viewtopic.php?f=30&t=86643

This will help a developer analyze why this occurs in your setup.

My setup is the space after artist as well and have not experienced any problems. A debug log may show what exactly causes this for you as you have space before - in other places in the Path.

The program fails to update the following file

D:\Musik\Klassik\Cleveland Quartet\Schubert - Forellenquintett D 667
\01 - Forellenquintett A-dur D.667 - 1. Allegro vivace.flac

with a CD cover, because it ignores the blank after "Schubert" in the path's name
with then trying to update a file which can't be found:

D:\Musik\Klassik\Cleveland Quartet\Schubert- Forellenquintett D 667
\01 - Forellenquintett A-dur D.667 - 1. Allegro vivace.flac

Screenshot: https://www.magentacloud.de/lnk/LLhJG8yP

Version 5.0.1.2433 under Windows 10 20H1