Alright, so I somewhat have solved the problem. I don't know why I didn't try this before, but after using a different thumbnail it worked flawlessly. The first thumbnail I was using that was causing the issue was a gif that I assume was to large for steam/crowbar to handle, so once I used a regular old png it worked fine. The gif was 17.2 mb if that is any help.

Also, I am using Windows.

Yes, that works just fine, I'd just like to upload through Crowbar if I can for the better thumbnail support

Steam is "not responding" when Crowbar tries to upload, and when Crowbar says "Error" steam totally closes without any error or crash message, and yes, Crowbar stays open and I am able to continue to use it (I am unable to use the publish feature obviously because Steam is now closed).

My file size is 281 MB

The quota below the publish button just says "unknown"

The dev version still caused the same issue with Steam crashing. The log at the bottom (if that's what you mean) still says "ERROR:" with nothing after.

I am trying to upload a piece of L4D2 content to the steam workshop, but keep running into a problem in Crowbar. While Crowbar is very clearly able to connect to steam as it shows all of my other workshop content in the publish tab, when I try to upload a new piece of content, Crowbar gets stuck on "Processing content for upload." for a few minutes, until steam crashes and Crowbar just says "ERROR: " (with no following error message). Any help?