Just as a note: We use Proxmox together with UCS, in various combinations, from UCS3 up to UCS4.2, from PM3 up to PM5 (at time of writing PM5 is still beta), for several companies of various sizes, with and without support from Univention and/or Proxmox.
We never experienced any problem with the UCS installer, no matter textmode or graphical installer was used for installation as virtual machine in Proxmox.
So please check your UCS installation media (md5sums are ok?) or your proxmox configuration, or the configuration of the virtual machine (sufficient ram/cpu/hdd-image?)
https://docs.software-univention.de/installation-4.2.html#appliance:create:virt
https://docs.software-univention.de/installation-4.2.html#appliance:installbase
https://updates.software-univention.de/download/ucs-cds/ucs4.2-0/
http://updates.software-univention.de/download/images/md5/
Proxmox-specific help:
https://forum.proxmox.com/ or https://pve.proxmox.com/cgi-bin/mailman/listinfo/pve-user or https://www.proxmox.com/de/training/video-tutorials or (if you own a subscription) https://my.proxmox.com/ or (if you need a local partner) https://www.proxmox.com/de/partner/reseller
hth, regards Lutz