I am using rdiff-backup to backup all important files for about 10 years now.
Target is an NFSv4 mount. Again in 5.14.21-150400.24.49-default this does not
work properly any more: read and writes seam to be affected. When booting back
to 5.14.21-150400.24.46-default everything works fine again which eliminates
all reasons related to python or rdiff-backup itself. 
My kernel is set to preempt=full.
Unfortunately the error happens quite discretely: no error messages in dmesg or
journal appear.
In some 15.x version NFSv4 too was broken by some kernel update once: directory
listings (which in my case contain about 15k entries) suddenly where truncated.
I hope this isn't some recursion?