Time Machine doesn't unmount anymore

[Sciuriware]

Hello,
ever since I upgraded to 10.11.6 one of our iMacs has a problem:
Time Machine mounts the Time Capsule (and after one hour the second hard disk) but never
releases them. A reboot cleans up, but after 2 hours the situation is the same.
So there is /Volumes/Time Machine Backups and /Volumes/Time Machine Backups 1

Our other iMac and MacBook do not have this problem, although running 10.11.6 too.
What could be the cause?
;JOOP!

 

[Sciuriware]

Hi all,
I hate to tell you that the problem has gone since I installed 10.12 Sierra.
So, I'll never understand what caused it.
Whatever ....
;JOOP!