Sync setup fails: "Failed to list a folder. Error. Unknown error"

I have multiple Nextcloud instances (company and clients). When testing the sync client with a new client deployment I cannot complete the setup stage “Add Folder Sync Connection”, with the error as per subject (and see screenshot below). The same desktop client is happily working with a different Nextcloud instance.

The web UI works fine for the user account.

So far have tried: deleting and recreating the account (both on the desktop client and on the Nextcloud instance) and upgrading the client (was still the Owncloud branded one) and checking with an early adopter from the client that the desktop sync is working for them.

Differences between the server with which the desktop client works and the one that doesn’t: working instance uses internal auth and is a single server installation, the other uses LDAP/AD and is multi-server.

All input welcome!

“Create Folder” works, but then the error message returns and the folder isn’t available to choose as a target;
the desktop app correctly reports the amount of data in use by the test account on the server.

There is a general howto for troubleshooting the client:

Especially the part with logging is important. Also check your server logfiles.

F12 key useful, had previously looked for a log out in the UI. Although given:

the client works with another Nextcloud instance
another user working with this Nextcloud instance can use the desktop client without issue

I wondered what the unique setup combination could be.

Log gives me (on attempting to “Add Folder Sync Connection” and encountering error described in original post)

12-03 12:52:43:356 9177 OCC::AbstractNetworkJob::start: !!! OCC::LsColJob created for "https:// + “/” "OCC::FolderWizardRemotePath"
12-03 12:52:43:867 9177 OCC::LsColXMLParser::parse: Invalid href “/nextcloud/remote.php/webdav/” expected starting with “/remote.php/webdav/”

Webdav address does seem to be with or without the /nextcloud/ part in visiting (/nextcloud)/remote.php/webdav/ in a browser returns the message:

This is the WebDAV interface. It can only be accessed by WebDAV clients such as the ownCloud desktop sync client.

Additional: noticed in passing in Personal > Sessions:

Sync client - Linux an hour ago

(checking details, 4 of these a minute when client is up)