It was indeed an older database, that we cleaned.
All the 'old' Superuser numbers were replaced to the correct new Superuser number
But the Events are created and eddited by two administrator's (not the Superuser), that is why they didn't see the venue's!
Again I learned someting (i'm glad i can learn every day)
So for the futher: if you can't see the list of venue's check these setting:
general settings => edit event => Owned venues => YES / NO (YES is default, select NO when using several admins!)