Installed plugin to empty cache: As a result the right pane which display the blocks of the selected page was complete empty. When I tried to select a page to edit, nothing happens ... So I could not edit any pages.
Because I had to fix the problem in this weekend ... to get my page live again ... I made the decision to remove my CMS directory and uploaded the CMS directory of my local test environment and connected the database again from my live server. Got my media back and all works fine now. So for now, no plugin and system updates on my live server. Will try to figure out which update caused the disapearing of media and the (No) edit pages problem on a test environment.
My project is now working fine ... local and on the live server of my hosting partner.
There were some unexplained problems (missing images) but I solved it in the following way.
I updated all my plugins (except the RainLab Pages plugin) and the system first.
Then I updated the RainLab Pages plugin.
At that point I could not edit existing pages (only add new pages).
When I noticed that it had something do do with browsers, I did the following:
In Safari I disabled the AdBlock extension and loaded the backend again while pushing the reload button and choose for the option "Reload Without Content Blokkers". Then I could load and edit my pages again.