OK I (you) did it!
Correct code looks like this:
Private Sub Form_BeforeInsert(Cancel As Integer)
Dim vLast As Variant
Dim iNext As Integer
Me![RIMYear] = Right(Year(Date), 2)
vLast = DMax("[RIMINC]", "[RoadIM]", "[RIMYEAR]=" & Right(Year(Date), 2))
If IsNull(vLast) Then
iNext = 1
Else
iNext =...