Well... I'm not really sure why it wouldn't be there, but compatibility issues between ec/Ubercart are as good a guess as any. It's hard for module packages as large as these to totally clean themselves out when removed/uninstalled, so it could be some leftover conflict. There's not much to really do besides poke around in the database, clear out the cache tables, and see if you can turn anything up.

Sorry it's not an exact science... is it too late to consider just starting from a new DB?