In an Advanced Availability setup, in order to allow the Background and the Standby server to access the .ebr files, these files are in a shared location.
After mount the shared location with NFS mounts instead of UNC mounts , the problem and errors reported get fixed.
Regarding this, note following point in the documentation, section "Considerations for Advanced Availability Configuration":
"You require a remote database server and a server to share knowledge Management index files and import/export files, archive purge output files, and attachment repositories. To allow the background and the standby server to access these files, a shared location is required. Linux and Unix installations can use NFS mounts. UNC support has been added for Windows installations."