I would like to ask if anyone has the same problem and might know a solution:
Using CHR on Proxmox and the standard Linux bridge on the Proxmox hypervisor leads to the problem that EoIP is not working.
So far I am able to torch that GRE packages are not passed through the Linux bridge to the CHR VM.
Therefore I switched from Linux bridge to Open vSwitch and suddenly GRE is passed and EoIP works. But other problems appear, so I would like to stick with the standard Linux bridge.