What would i do now to check what cause the Problems?
switch from Arch package Version to AppImage (its most easy, since all credentials are safe and wont be deleted. Especially not the file /home/<USER>/.config/Nextcloud/nextcloud.cfg.
if that won’t help i woud try to use X instead of Wayland. Some Distros allow to switch between X and Wayland at login-screen. In Ubuntu community Wayland is well known for cause issues with some Software
but well you may also wait for someone’s response using same config as you use, Means Arch, KDE, Wayland and Client version 33.0.7 from Arch Package