Marchalling problems with UDT

An ActiveX control creates an instance of an ActiveX exe (early bound).
The ActiveX exe contains a public user defined type, and uses it as parameter in a public method.
On one (and maybe more) Windows 98 machine the control generates a runtime error 429 at the moment of the call to the method.
This has usually to do with a component that isn't correctly registered, but we can exclude this cause. Other methods in the ActiveX exe can be called with no problems.
We have tried unregistering the Activex exe, and reregistering it, but this didn't solve the problem.

There is a known issue on NT4 with marchalling a udt (solved with sp4), but the problem is with Windows 98.
There is also a known issue with marchalling a udt when you use late binding, but in this case early binding is used.

We have tried if installing DCOM98.exe would solve the problem, but we get the message that a newer version was already present, so we cancelled that.

If anybode has an idea about what can cause the problem, I would be glad to hear it, because redesigning the solution is not an option at the moment (it was about a year work).