@fervous Problem solved!
Here’s how I tracked it down. I was looking at WooCommerce > Commission for a completely different reason, and I happened to notice that the error appeared on two specific orders from Feb 5 (Lilly) and Jan 22 (the other fictitious vendor who was having the issue, Jake). Preceding the error were the words “invalid order”. These happen to be the first two test orders we ever made.
I couldn’t remove the orders from there, but I tracked down this thread (https://www.wcvendors.com/help/topic/reversed-commission) with Ben’s advice on how to delete orders from phpmyadmin. It worked – the two interfering orders are gone, and so are the errors on the vendor dashboards.
As this error was discussed on a few separate threads, you may want to direct everyone here.
So the problem can be solved, but the question remains: Any idea how this happened, and how to prevent it in future?