i got it now!

the correct type declaration is:

Code:
Private Type RASDIALPARAMS_W
   dwSize As Long
   szEntryName(RAS_MaxEntryName * 2 + 1) As Byte
   szPhoneNumber(RAS_MaxPhoneNumber * 2 + 1) As Byte
   szCallbackNumber(RAS_MaxCallbackNumber * 2 + 1) As Byte
   szUserName(UNLEN * 2 + 1) As Byte
   szPassword(PWLEN * 2 + 1) As Byte
   szDomain(DNLEN * 2 + 1) As Byte
End Type
i only need +1 because the Array starts with Zero!

...and the correct size for the API call is 2096 !

Now everything works!

Thanks you for your help to point me in the right direction!