We have been seeing these same symptoms after a content database upgrade and running WikiPlusUpgrade. What we have found is the following:
1) Initially, the Tag Cloud web part is listed in Miscellaneous but there is not Kwizcom group in the web part gallery. Adding this web part to the page still results in an error
2) Reactivating the "Kwizcom Tagging Feature" at the site collection level creates the Kwizcom group
3) We are then able to add the Tag Cloud web part to the page and it renders.
We dont' have a huge number of wikis so we could manually make these fixes but it would be nice if the WikiPlusUpgrade tool could correct these.