Problem solved!
The 9.5.1 update appeared to be installed on an external drive instead of the original system-drive.
That's why the app.store said that the update already was installed and my iMac executed the old version.
But posting a question in this group often leads to the answer spontaneously.
This is what I did.
Looked up iPhoto in spotlight
( there appeared to be two versions!! One on the system drive and one on an external drive)
- I first renamed the iPhoto.app (9.4.3.) on the systemdrive,
- then I replaced the 9.5.1. app from the external drive to the systemdrive.
- tested the mew version, which seemed to be OK
- deleted the renamed (old) version
Happy newyear to myself!!