Sounds like any exclusive index access (like the one required to drop or rebuild it) seems to fail.
Did you manage to unload/load the table?
Concerning the amount of threads: I am not aware of any dependency between "index present" and "number of threads".
Really, we miss information here and the support incident is - in my eyes - the best way to find all required information.
Would be nice if you post the result of that!
- Lars