I just stumbled across this post through a google search.
I've opened a ticket with Icedrive support in december 2020. I've provided them with the exact error message for Joplin (i also had difficulties with Enpass).
Their final answer came after a few days:
We're sorry to say that as of Jan 1st we have decided to drop WebDAV support, in some reason due to the problems you are experiencing above - It is a very old protocol and many 3rd party apps just dont use it properly, or use it differently. It is best not to support it since it produces so many problems for so many users. If you are looking for sync, our own software will do what you want and more, without the problems you have
It's incredibly bad customer support, especially since on their own website they (still) state:
Icedrive fully supports the WebDAV (World Wide Web Distributed Authoring and Versioning) protocol.
It seems Icedrive doesn't really care about WebDAV or about their customers very much.. Ashame you only find out after subscribing.
So far i've tried a lot of things (create folder structure etc) but nothing seemed to work. Another app (Orgzly) managed to create a folder structure and save notes without problems though (through WebDAV on IceDrive).
Edit: One more addition, i just setup Genius Scan (android app) to sync over WebDAV. This works flawless, it even pulls the folder structure and let's you browse through. Possibly it would be something that Joplin could improve?
Edit 2: I have just installed the latest version of Joplin and tried once more. When I test the configuration in the setting i get a succes message (already better than before), however when trying to sync it shows the following error:
Completed: 26/02/2021 15:14
Last error: Error: PUT locks/exclusive_desktop_227e5505875b496ca256b12221f29df0.json: Unknown error 2 (429):
Same error as the OP. The first error (MKCOL locks is no longer occuring). I've tried syncing to root, and syncing to a pre-created folder. But to no avail so far.
Edit 3: Joplin did manage to create two folders when syncing to root. One is called 'Locks', the other is called 'Temp'. Both folders are empty.