@MarcDriver Ok, ignore my last post about trying an older version. I found that kDrive offers a 30 day trial of kDrive solo, so I have set up account for testing.
I am able to sync Joplin data to kDrive in both v3.4.12 and v3.5.13 of Joplin desktop without any issues. Therefore there aren't any Joplin client side changes which is causing this issue. This means either something has changed on your specific kDrive account, or the Joplin data on your account has somehow got corrupted. As Infomaniac will not offer you support with this, the only thing I can suggest is to follow this process How to fix synchronisation issues and start over, which will replace the server data with a known good copy of your local data