Uploading to Steam Workshop not working?

I have created many mods over the last years for this game and yesterday I wanted to upload yet another mod.
However I get the following error while uploading, even after making sure the previewimage was less than 1 MB.
Is there a server side problem with the workshop at this time?
Or any suggestion on how to resolve this?

// Steam, after clicking upload

Sorry!
An error was encountered while processing your request:

There was a problem accessing the item. Please try again.

// Mod Manager, after clicking upload

Steam Upload: Error

An error occured while trying to submit to the Workshop. (9 - FileNotFound)