Mostly posting this so people running into the same problem as I know what to do. When I tried to change the background image of a level in game, it would let me select a workshop image as intended. However, the download of such an image allways got stuck in the first step (initalisation). After some time (maybe 15 seconds?) a cancel button would appear, but no matter how long I wait, the Download would never actually start.

The Problem here were my Steam download settings: I had limited automatic downloads to a specific time frame. As soon as I extended this time frame to include the current time, everything worked as intended. So if you have the same problem, try to enable automatic downloads!

It's kind of annoying I have to change my settings to use this feature, but I am not sure how much can do about this as a developer. If you could change the download of the workshop item to count as manual (which it is?), thus ignoring settings for automatic downloads, it would be perfect. Should that not be possible, it would be nice if you could somehow notify players about the need for changing the settings if the download goes wrong. Maybe when you display the cancel button also display some text about this?