It seems that the TV's are loosing connection with the Philips Server because of some registration issue. I have two Philips TV's at home, and with both I had to register them again within a week.
After looking into several forum discussions which were not covering for 100% my problem I decided to do the following:

1. Clear the NET TV memory (you find it under Setup--->Network settings.
2. Go back to normal TV screen
3. Switch off with the remote control
4. Pull the electricity cable out of the wallsocket (yes, I know it sounds stupid, but it worked with me)
5. Wait a few minutes before putting the cable back into the wallsocket
6. After the Philips logo has appeared and disappeard wait a minute
7. Switch on the TV with the remote control
8. Wait five minutes
9. Go to SMART TV or NET TV.
10. The TV will ask for a registration, follow the instructions on the TV and in the e-mail which will be sent. After that it's working again.

I have the feeling that Philips is loosing/deleting the credentials, or that they want us the register the TV once a year to check if the account is still being used.

Maybe it will help others with the same issue

b.kuyper