I don't know if it helps, but doing a search in comes up with a number of applications that are suffering the same error, not just phpList.
The have a thread on the issue, there's one on the vBulletin forum too, OSCommerce as well.
I've looked at the MySQL forum thread, but have to admit been totally flummoxed!
Just found a post from the second page of Google results which I can understand. "Restarted MySQL and the problem went away". Might that help?