I have good news for you: I managed to delete things, and now campaigns work again!
The really important folder to look in is “\userdata<SteamID>\813780\remote” which is the steam sync folder for age.
In there you’ll probably find a bunch of large save-game files. Replace them with zero size files. Deleting is NOT enough - steam will helpfully keep restoring them if you delete! That can be done in a variety of ways, eg:
- Open your save game file in a text editor and just delete all the contents then save
- make a few empty text files and then rename them
- using the powershell “clc” script
Make sure that you end up with a newerlast modifiedtime.
When you turn Steam/Age back on, you’ll get a Steam Cloud conflict, which will allow you to replace the (large) server files with your (empty) local ones.