[SOLUTION] [rpc.svcgssd] „WARNING: error writing to downcall channel /proc/net/rpc/auth.rpcsec.context/channel: Operation not supported“

If you use NFSv4 with Kerberos krb5 gssapi authentication mechanism on Ubuntu Jammy 22.04, 
you could possibly get this error message, after the upgrade:

[>>] systemctl status rpc-svcgssd.service  #cat /var/log/syslog
rpc.svcgssd[24866]: doing downcall
rpc.svcgssd[24866]: WARNING: error writing to downcall channel /proc/net/rpc/auth.rpcsec.context/channel: Operation not supported

Solution:
Install the linux-modules-extra kernel modules.

1. Kernel Version Lookup:
>> uname -r
5.15.0-1022-raspi


2. Search in apt for the linux-modules-extra

>> apt-cache search apt-cache search linux | grep extra | grep 5.15.0-1022-raspi
linux-modules-extra-5.15.0-1022-raspi - Linux kernel extra modules for version 5.15.0 on ARMv8 SMP


3. Install the linux-modules-extra for YOUR kernel-version! For me it is a kernel on the raspi.
>> apt install linux-modules-extra-5.15.0-1022-raspi


4. Reboot -> Done
>> shutdown -F -r now

You did it! Now you can use Kerberos with NFSv4 on Ubuntu 22.04 Jammy. ;)

Sources: https://mailman.mit.edu/pipermail/kerberos/2011-May/017165.html