MongoEngine & Pymongo Error when save (any) document to collection

Question

I receive the following error when trying to save a document to a collection:

mongoengine.errors.OperationError: Could not save document (assertion src/mongo/db/storage/mmap_v1/btree/key.cpp:626)

The collection (and the db) exists and I've tried to empty it, remove any index in it, and nothing works: I keep receiving the same error.

Answer 1

After doing: sudo mongod --repair everything works normally.