Yup, it seems like there is a limit. I don't know if it's the number of items, or the total size of items:

From here:
https://partner.steamgames.com/doc/api/ISteamUGC

"k_EResultLimitExceeded - The user has exceeded their Steam Cloud quota. Have them remove some items and try again."

Huh. That is quite unfortunate.

I've deleted everything but the error still happens. Is there anything that you guys can do, or do I have to contact Valve?

I contacted Steam Support and they said they can't do anything. I deleted the only remaining thing I had left, which was my one submitted level and now when I load it locally it says "Update" instead of "Upload" and clicking it fails immediately with a "k_EResultFileNotFound" error in the player.log. I also still can't upload any new objects or levels, with the player.log file still saying k_EResultLimitExceeded.

Are workshop items not getting deleted properly? Can you guys just clear whatever workshop files may still be lingering? Is there some sort of tracked file size total that's not getting lowered when workshop items are deleted? Is this a bug with Steam?

We keep the workshop id for the level around in your local copy, so you can keep updating the level. That's probably why it's saying "Update" instead of "Upload" - because we aren't handling the case where you've deleted levels through the Steam client.

If you Save Copy of a level, that copy will no longer be tied to the deleted workshop level, and the button should switch back to Upload.

But of course that doesn't solve your real problem. Did you contact Steam support after you deleted your levels via the steam client (and/or website) and they still said there was nothing they could do? It would be useful to tell them that you're still getting the k_EResultLimitExceeded error code, despite deleting your workshop items.

I contacted them after I deleted everything and rereading it, I got something that felt like they read the word "workshop" and copy/pasted some "we don't offer support for creating workshop content" response. I've responded now, as I was waiting for your response here before I continued. I told them again that it's a data issue and not a creation issue.

I have seen other people are having trouble uploading workshop content too. I know nothing about how the workshop is implemented, but are there any settings in the backend that you developers have that could be related to this issue...?

Ok, we've just made some changes on the back end to increase the Steam cloud quota (which I think just the object/level thumbnail files count against). Can you try again?

I was able to upload a new blank level now. Thanks for working on this!

Are there any numbers related to the size of user submitted files? I should have nothing currenly uploaded. Maybe by not deleting the files through Superliminal itself, it's causing a desync?

The only parameters we can tweak are the cloud quota and the maximum number of cloud files (which I've also bumped up). But workshop content itself (other than the preview images we generate) is not included in that quota or file count.

So basically the cloud quota applies to the game's save files (there are 5 tiny files we use, if I recall correctly) and the preview images for your own workshop content.

It's entirely possible that deleting items through the steam client or website doesn't reduce the quota used up for the preview images, we'll need to look into that.

btw, some more log files that might be useful to diagnose any future issues are here:
"C:\Program Files (x86)\Steam\logs\cloud_log.txt"
"C:\Program Files (x86)\Steam\logs\workshop_log.txt"