It’s a known issue, which also appears in version 5.00.02. According to NhatMinh (see link), upgrading to the latest version, 5.00.03, has fixed the issue for him.
If you can or want to do this update, you can try.
Otherwise, 4.20.03 is the latest update for Next 4, so if you won’t upgrade to 5.00.03, try to remember what exactly you changed in the model, maybe you imported some component that triggered this problem, if you say it worked before.