I have now done something more drastic.
- First I turned of all synchronisations, turned iTunes Match of, de-authorized my computer, etc...
- Then closed itunes and used an application called 'App Cleaner' to uninstall iTunes from my system (you have to select it manually in the Applications tab, drag and drop won't work).
- I downloaded iTunes for Mac (latest version) and re-installed
- After a restart of the system I logged back in to everything and re-synched all my devices.
Until now the problem has disappeared...