ok, fixed by myself.
The link in the vCD Release notes links to an incomplete KB article.
vCloud Director 9.5 for Service Providers Release Notes
Note: If you use NSX for vSphere as a load balancer, you can increase the number of headers in the response only if using version 6.4 or later. See https://kb.vmware.com/s/article/52553.
This KB only changes
"lb.global.tune.http.maxhdr=1024"
https://kb.vmware.com/s/article/67696 show the correct changes
lb.global.tune.http.maxhdr=1024
lb.global.tune.bufsize=65536
After setting the bufsize parameter, all is working