@youteeee Have you been able to fix this? I think the developers screwed up the login flow and only support the new flow. I’m using Nextcloud 15.0.11 and I’m still using a special ownCloud client build (2.4.3) which supports h2.
As with many other parts of Nextcloud the priorities are rather on new features than fixing and stabilizing the core stuff. (I still think that Nextcloud is a great product (starting usually around N.0.5), but the dev process is one of the worst I’ve ever seen.)