Thanks OT for your comments. Let me answer to yours (and Larry"s) questions as follows:
What is an AirCapsule? Do you mean a TiimeCapsule by any chance?
Yes, of course, I think lukassek meant a TimeCapsule. At least this is my case.
If so how are you connected to it?
Via WiFi
Your library must be on a direct connected drive (fast wired connection) and the volume your library is on must be Mac OS extended (journaled)
This is NOT correct. Until now I have been perfectly working with ATC, and Photos worked just fine. Your information is wrong, sorry.
what fixes have you tried?
None, apart from restarting both the program and the Mac. This is my problem, no one seems to have this issue because there is no other forum about this. All other comments are about importing libraries from iPhoto or Aperture. But in my case there is nothing to import. Until yesterday I have been using my Photos library flawlessly, and since 10.10.4....problem there.
is it a referenced or managed library
It is a managed library (i.e. I am not referencing to pictures stored somewhere else, but have ticked the box of "copy items.."
Maybe its just lukasssek and me having this issue, but...hopefully there are more, so that Apple (or whomever) can fix the bug.
Thanks in advance for any tip.