Can't adda a cover to flac-file on my NAS-server

[LyricsLover]

This is not a bug. This is the expected behaviour - especially for big flac files.

If you add a Cover JPG to a *.flac file it has to be written completely new (in most cases).
This can take a pretty long time and depends highly on your current local network speed and the speed of your NAS hard drives. This seems like "the system hangs".

As you already have found out:
Just copy your *.flac files locally, add the covers and then copy them back to your NAS.

Similar threads:

Writing Album Art to FLAC very slow over LAN

I have all my CD's ripped to FLAC file per album. Each FLAC file varies from 200MB to over 500MB. They are all stored on my NAS which has a Gigabit connection. I think because the NAS is rather full, it writes at a speed of around 3MB/s whilst it can read at 20MB/s. I'm attempting to Album Art to each FLAC and over the network, MP3Tag can take up to 20 minute to write the image file into the tag for just one album. If the file is stored locally, it's just a second or two. Is this expected? …

and

Mp3tag rewriting FLAC files when adding cover art?

It's not using a temporary file in all cases, because under certain conditions, padding can be utilized so that writing edited metadata back to the FLAC file does not require rewriting the entire file. If rewriting is required, then a temporary file is used by Mp3tag.