If you run into a vague error message when submitting or downloading a workshop file the best resource is usually Steam's own workshop_log.txt file. There should be a description of any recent errors near the bottom.
When playing through the Steam library it can be found in your Steam install folder, typically here:
- Windows:
C:\Program Files (x86)\Steam\logs - macOS:
~/Library/Application Support/Steam/logs - Linux:
~/.steam/steam/logs
For servers hosted using steamcmd it can be found in the logs folder of your server.