IEF plugin does not exist

This must have been a Thunder bug. The inline_entity_form plugin doesn't exist, but it used to. That was the original machine name for one of the field plugins, going back to the D7 era. I think it was what became the Complex widget, but I'm not certain. If Thunder included configuration for an entity_form_display that used the inline_entity_form widget, then that configuration would caused the error described after IEF changed the widget machine name. Deleting and recreating the field would have solved the problem exactly as described in comment #2, though I have no ready explanation for those of you for whom that didn't work. I suspect that simply re-selecting and saving the widget that's used in the form should have repaired it too.

In any case, Thunder no longer includes any configuration that uses the old plugin name. So I don't expect that it has the problem any longer.