Your description is very vague, so here's a wild guess at what's going on:
- a WebDAV mount was setup in DAVx⁵
- DAVx⁵ then made this accessible to other apps via Android's Storage Access Framework (SAF)
- a backup app was granted access to this SAF location and did backups there for a while
- but recently "the storage location was supposed to be reset"
Meaning the backup app's connection to the SAF location is now invalid and you need to remove and set it up again in that app? But instead you set up the WebDAV mount again in DAVx⁵, which didn't help?
I could be totally misunderstanding your post... in that case, try to clarify so we can help you better.