I have some good news for you: our Rocky8 VMs have been updated to 4.18.0-553.134.1.el8_10.x86_64 (and some even to the latest 4.18.0-553.141.1.el8_10.x86_64) kernel on 24th June (3 weeks ago).
Since then the problem disappeared completely and today we continued our V2V migration from VMware to XCP-ng.
Thanks for all your support!

We have checked our kernel versions, some are very old:

     10  4.18.0-553.109.1.el8_10.x86_64
      3  4.18.0-553.117.1.el8_10.x86_64
      2  4.18.0-553.120.1.el8_10.x86_64
      4  4.18.0-553.16.1.el8_10.x86_64
      2  4.18.0-553.22.1.el8_10.x86_64
     12  4.18.0-553.30.1.el8_10.x86_64
      2  4.18.0-553.34.1.el8_10.x86_64
      4  4.18.0-553.36.1.el8_10.x86_64
      1  4.18.0-553.40.1.el8_10.x86_64
      2  4.18.0-553.47.1.el8_10.x86_64
      3  4.18.0-553.51.1.el8_10.x86_64
      2  4.18.0-553.54.1.el8_10.x86_64
      3  4.18.0-553.56.1.el8_10.x86_64
      1  4.18.0-553.58.1.el8_10.x86_64
      4  4.18.0-553.62.1.el8_10.x86_64
      2  4.18.0-553.63.1.el8_10.x86_64
      3  4.18.0-553.69.1.el8_10.x86_64
      2  4.18.0-553.74.1.el8_10.x86_64
      1  4.18.0-553.81.1.el8_10.x86_64
      1  4.18.0-553.83.1.el8_10.x86_64
      2  4.18.0-553.87.1.el8_10.x86_64
     16  4.18.0-553.89.1.el8_10.x86_64
      4  4.18.0-553.94.1.el8_10.x86_64

The OS team will do the kernel upgrade and I will come back with the result. Versions .109, .117, .120 did not fail yet in our environment. We have high hopes!

Is there any specific reason to limit nbdkit server to threads=1?
spanwNbdKitProcess
According to our tests significant improvement (up to 4 times) could be achieved when increasing thread count.
Is there any plan to make it an user configurable parameter (in xo-cli for example)