To Be Tested: To prevent this error from happening again, if Time Machine is in-progress doing a backup, try doing a "Stop Backing Up" command in Time Machine before moving your MacBook to another location off-network.
I am not sure exactly how this situation occurs, but here is my theory: After disconnecting the MacBook and reconnecting to another network (moving my MacBook to another location while traveling), the connection to the synology resource: mac-backups (used by Time Machine) did not get properly disconnected, so when the machine was reintroduced into the network, another new connection was established, even though the existing connection was still present, creating a conflict and the subject error message. Mac Network share always mounting in Finder, but never for Time Machine (High Sierra 10.13.4) Mac Cannot backup Time Machine to Synology NAS due to problem with username or password. Time Machine backup begins (circling counterclockwise) and you no longer get the above error message. On your MacBook > Time Machine icon > Start Backup.One user connected to AFP > Resource: mac-backups.
The issues seems to be that there are two or moreĬonnected users both connected to AFP > Resource: mac-backups. Resource: mac-backups, press disconnect (red circle with white The backup disk image “/Volumes/mac-backups-1/machine-name.sparsebundle” is already in use." and you are using a Synology DiskStation DSM 5.2 (NAS) as the destination for Time Machine backups, the following steps resolved the issue: If you get the error message "Time Machine could not complete the backup.