The backup on Server is already in use I have OS X Mavericks with the Server software installed. The server machine is an iMac with a Drobo attached via USB. I use the Server Time Machine service to create a backup server on the network, using the Drobo as the storage device for backups.

I am currently backing-up two Macbooks to this Time Machine Server. Some backups will succeed but every other day or so, I will get an error on both laptops that the backup failed with the following reason: The backup on Server is already in use.

I have only been able to clear this problem by restarting the server. 

Does anyone know why this might be occurring and what I might be able to do in order to prevent it from happening?

Thank you.
Just for the completeness of this issue. If you land here and do not use MacOS Server, but a Synology DiskStation, do the following:


Login to your DiskStation as root
Open the main menu
start the resource monitor
switch to connected users
select the blocked user and press disconnect