That seems pretty weird, I appear to be having a similar issue in the same sort of situation, but a different message upon going to load the Wisps in. Your wisps don't have the Entangle Gold Mine ability at all, do they?
It might be possible that given our problems are similar but different, with the comparison of notes we could eliminate both problems.
UPDATE: I found that creating the worker unit based on the Wisp itself appears to trigger the same message that you have been encountering. As of yet, I don't know why it is doing that, but it does seem to be the same result as what is happening for you.