Can't connect to shell in web interface

[HaZu]

Hi, I set up a new Proxmox Server on a Dell R7515. So far so good, but if I try to open a shell in the web interface, I get the below error message:

failed waiting for client: timed out
TASK ERROR: command '/usr/bin/termproxy 5900 --path /nodes/ddcrlbc888 --perm Sys.Console -- /bin/login -f root' failed: exit code 1

Same goes for a VM shell. Connection via SSH works fine. Any Idea how to debug this? My guess is it has something to do with the network. Using my Client to connect to my home lab, all is good.