Backups fail with error messages stating that the SnapAPI module is not loaded for the running kernel.
In dmesg/messages, you can find these errors from SnapAPI: "make_request_fn is NULL."
Backup agents BEFORE C21.09 release had a version of the SnapAPI kernel module (versions up to 0.7.140) that could not compile successfully on kernels 4.18.0-315 or higher (in the 4.18. series).
The new version of the SnapAPI module (0.7.140) that ships in the agent C21.09 release is now compatible with these kernels. Update the agent to C21.09 (or later) for the kernel module to compile, load, and function fine out of the box.
NOTE: This most likely applies to RHEL/CentOS derivatives/replacements such as AlmaLinux and RockyLinux, as well, as they share the same or essentially the same kernels, by design.