Nevermind I found the issue,correct constraint is:
[DemoApp.RegistratieEvent/DemoApp.Event/Date >= '[%BeginOfCurrentDay%]' and DemoApp.RegistratieAccount/Administration.Account [id = '[%CurrentUser%]']]
Thanks
I think you are missing the Module name before RegistratieAccount. You also don't need the extra [ when accessing the account id.
[DemoApp.RegistratieEvent/DemoApp.Event/Date >= '[%BeginOfCurrentDay%]' and DemoApp.RegistratieAccount/Administration.Account/id = '[%CurrentUser%]']
or
[DemoApp.RegistratieEvent/DemoApp.Event/Date >= '[%BeginOfCurrentDay%]']
[DemoApp.RegistratieAccount/Administration.Account/id = '[%CurrentUser%]']
I believe both of these are equivalent.