I'm not sure if that belongs to the topic, but I'm currently migrating from Owncloud 10.5 to Nextcloud 19.0.2., and have the same problem with Enpass. The username and password are definitely correct, and I still get a 401.
However, I have found that if I bypass the HAProxy connected in front of it, the synchronization works immediately. The HAProxy is in http mode and has so far run with the Owncloud without any problems.
I'm still looking for the problem, but I'm afraid that it could be an interaction with the Nextcloud, since I'm now on the standard configuration of the HAProxy and really can't do much more.