I'm getting an error message whenever I try and turn on dynamesh on this one subtool. The subtool is super simple, it's just a cylinder that has been exported from maya. The cylinder had been scaled up and moved away from the origin, but then it had it's transforms frozen and history deleted. I've tried searching the web…
But even everything is fine with the scale and the symmetry and the pivot, this dumb pop-up appears when you try to dynamesh an object with SpotLite(suddenly!) is turned on. Just hide it by pressing Shift+Z and then dynamesh the subtool.
I also have had this problem and it drives me fucking nuts and made me do things in a very roundabout way. It has stopped happening, but I think what I would do would be to save that obj as a separate tool, reboot Zbrush, open the tool and Dynamesh it, and then add it to my sculpt. Super annoying.