Remote Desktop Manager webdav unauthorized 401

We keep getting unathorized errors in logs on webdav iis server and in the logs of remote desktop manager free.

If i connect using the net use method all works.

example log in IIS: 019-10-30 09:25:05 W3SVC2 IWF051 10.0.0.51 OPTIONS * - 81 - 10.0.0.38 HTTP/1.1 DevolutionsWebdavClient - iwf051.df0.lf.lan:81 401 2 5 1596 170 19

example log in Remote Desktop Manager:
[29-10-2019 18:24:53] PROPFIND /
[29-10-2019 18:24:53] 401 Unauthorized

What am i missing ?? or how can i get this working ?

Server is Windows 2012R2 with IIS