Recently I have started to get this error for a backup of my C drive - this backup has been in place an working for a long time.
"Unable to back up a locked volume encrypted with Bitlocker. Unlock the volume or turn off Bitlocker, and then try again"
The drive is the OS drive so is definitely unlocked. It seems to have started since I upgraded to the latest build of Acronis. At the same time it is also complaining a lot about locked files on other backup now where it never used to.