Have you checked it in the arschema ? Indexes should exist in both the tables 1)schema_index & 2) user_indexes. Following query will give you indexes which exists in schema_index but not in user_indexes.
select * from schema_index where not exists (selectindex_name from user_indexes where schema_index.indexname=index_name)
This is a clear case where the AR System repository is out of sync with reality, meaning that the index does not really exist.
The easiest way to fix this is to create an index yourself with the same name directly in the database.
I suggest you do this via a filter, as it will make sure the filter gets created in the right place with the right permissions.
After this, you should be able to drop the index via Dev Studio.
CREATE INDEX I401_536870922_1 ON T401 (C1)
Note that it does not matter which column(s) are used, as the important thing at this point is the name of the filter.
Best Regards - Misi, RRR AB, http://rrr.se