I ran into the same issue. I had to clear out entries in two files.
1. /etc/openmediavault/config.xml
In here, look for <mntent> tags. dev-disk-by-label-NAME was listed twice for me (with different UUID's, but the same fsname). I deleted one of the entires, and saved the file.
2. /etc/monit/conf.d/openmediavault-filesystem.conf
In here, I cleared out the identical references to dev-disk-by-label-NAME. In my case, I had two of these for FileA. I removed one of them and saved.
Code
check filesystem filesystem_srv_dev-disk-by-label-FileA with path "/srv/dev-disk-by-label-FileA"
if space usage > 85% for 5 times within 15 cycles
then alert else if succeeded for 10 cycles then alert
check program mountpoint_srv_dev-disk-by-label-FileA with path "/bin/mountpoint '/srv/dev-disk-by-label-FileA'"
if status != 0 then alert
Finally, reboot after making these changes. It should solve the issue.