Media browser shows content of one subfolder folder not of other subfolder

On my Synology NAS i have two subfolders under media.
Both contain mounted directories of another mediaplayer (used as external harddisk)
The content of one directory is shown, the content of the other directory is not.

At first the content of both directories were shown, but after reinstalling my router I had to re-mount the directory which content does not show up anymore. Restarting server or docker container does not help.

I cannot figure out how to solve this.

EDIT: when I leave the folder that does not show any contents an error message shows up saying :
expected str for dictionary value @ data[‘media_content_type’]. Got None