I can add, that it says “choose another location. Users/XXX/XXX is already used as a synchronisation folder.
I used this location for my old nextcloud that is no longer used and I installed a frech new Nexcloud on my server. So I don’t understand this message.
I have to say that also used this folder with Google drive (for my job). Is it a possible reason why ?
Open the folder Users/XXX/XXX in finder, press Command+Shift+. (to show hidden files).
Then delete the file named .sync_XXXXXX.db (or move it to a location that is not synched with Nextcloud).
This is the database of synced files from your old Nextcloud. When that file is not in the folder anymore, you should be able to add that folder to the sync-client. The database will then be recreated.
P.S.: when you are done, you may want to press Command+Shift+. again (to hide hidden files)
I would rename the old folder to something else. Then sync to a new empty folder, and don’t sync the same folder with two different sync solutions at the same time. If you still have other files, they might create conflicts. And two sync clients might get confused if they sync changes from both servers at the same time.
Then you can go through the old folder and check what you need to keep and what is not in your current one and move it there.