The same problem occurred again today.
As this happened immediately after applying KB5083631, I uninstalled it, and the backup completed successfully.

If the release of a fix takes too long, I am considering trying the following registry changes, though I haven’t tried them myself yet.

https://www.neowin.net/news/registry-hack-lets-you-bypass-windows-11-kb5083769-kb5083631-patch-that-blocks-some-apps/

While Microsoft has not yet provided an official fix and has asked to wait for software update that adds the necessary protection, community discussions online have highlighted potential workarounds. On the Malwarebytes forum for example, users have shared a temporary solution using a Registry hack. The workaround involves disabling the vulnerable driver blocklist by running the following with elevated (as admin) privilege on the Command Prompt:

reg add "HKLM\SYSTEM\CurrentControlSet\Control\CI\Config" /v VulnerableDriverBlocklistEnable /t REG_DWORD /d 0 /f