I accidentally wrote something and then deleted it in the upper section of Nextcloud web portal for a particualr folder. After this, the sync client started throwing this error message which I think the Readme.md file has to do with that section that you can add comments.
I tried disabling the “Show Richtext” optino and still get this error.
Also I tried by unsyncing the specific folder and it got fixed. So I deleted it, and recreated it…then synced it back. A lot more work than yours, but I think both of these methods are good solutions.