I was finally able to solve this issue.
There were multiple users experiencing the same issue.
It got resolved by UNchecking "OpenVPN > Client Export > Certificate Export Options > Use Microsoft Certificate Storage instead of local files.
Then "Save as default", export new installer en reinstall on client.
Don't know yet what the root cause is, but this solved the issue on alle of the clients.
Anybody here that knows what really is going on?
Thanks,
Thomas.