I plan to clone this bug to cifs-utils, as it is unclear which package has the bug.

After upgrading from samba 2:4.9.13+dfsg-1 to 2:4.11.0+dfsg-10 on a server machine, mount.cifs from cifs-utils 2:6.9-1
on a different machine fails to connect with "mount error(13): Permission denied".  Subsequently upgrading samba to
2:4.11.1+dfsg-1 did not help.

Downgrading samba to stable 2:4.9.5+dfsg-5+deb10u1 on the server fixes the problem.  Adding a snapshot version to figure
out what needed to be downgraded to get version 2:4.9.13+dfsg-1 was deemed not worth the effort, as my daily backup on
the client machine (which mounts the remote cifs share) was running fine when the server had 2:4.9.13+dfsg-1.