Adding song to iTunes Playlist stuck on “in progress” or error 11.444

So, I'm trying to make a playlist for a party this weekend, and it's horrible. iTunes more often than not shows "in progress" when adding one single song (this shouldn't take longer than 1 second, I recon) and finally, it will just show a ! with "Due to error 11.444".

I can't find anything about this error, and I have quite some songs left to add, so I'm hoping to find a solution here.

Edit: The same seems to sometimes happen whenever I restart iTunes after I can't successfully add a song, but then with "Making your iCloud music library available"

Edit 2: Sometimes it also goes as expected for at least a few songs, only to result in the same "processing" and then ! again.

Here are some screenshots, where I tried to add 2 whole songs to the playlist: