The thing is, I've seen that error before. It was during the Beta when they changed functions from returning icon paths to instead returning an image id and it has been fixed in Ace 3 since before the pre-patch rolled out.
So, one thing I noticed in the two stack dumps I received from users is "AceConfigDialog-3.0-63" in the path. I believe the minor version of that file in the current release of Ace3 is 61. So, my hypothesis would be that some addon is getting bundled up with a version of AceConfigDialog stamped with a higher version number than the current release, but which is missing the fix for the change to images. That version of the lib is getting loaded and my addon breaks as a result. I've asked one of the users who reported the bug if they could check every version of that file in their addons folder to see if one has a minor version number of 63.
Would I be right in interpreting things that way? Can anyone shed any light on this?