Console error -50 / beachball after editing song meta data in iTunes 12.9.5.5

Hi,

After I edit any song data in my library (comment, artist, etc), I get a spinning beachball for 3-5 seconds — everytime. This is making using my iTunes 12.9.5.5 library very slow and barely usable. I've checked the Console logs to see what's going on, and at the exact moment I commit the edit on the song info in iTunes, this is logged in the Console:

Assertion failure: err == (-50)

What does this mean, and how can I fix this? My iTunes library is on an external hard drive. Playing music works normal; the issue happen after editing meta data on a song in iTunes itself.

This still happens when booted in safe mode, and happens on two separate Macs (same OS / iTunes version; loading the same library).

Mojave 10.14.6; iTunes 12.9.5.5

Thanks.