I built a tree using AsyncTreeNode. The data for the nodes comes from a Java servlet using JsonArrays and JsonObjects. I also set the qtipCfg info into this JsonObject. The qtip shows up fine in both Firefox and IE. However, in IE, I get an error "Object does not support this property or method" (line 11, char 3562). The IE script debugger hits the ext-all.js.
Use ext-all-debug.js for development. At least then you'll be able to see the actual line it fails in - might give you/us more of an idea of what the error is.