Starting with its version 5.1, MongoDB disabled the legacy protocol used for communication since its beginning.
As a consequence, our mORMot client was not able to communicate any more with the latest versions of MongoDB instances.
Last week, we made a deep rewrite of mormot.db.nosql.mongodb.pas, which changed the default protocol to use the new layout on the wire. Now messages use regular MongoDB Database Commands, with automated compression if needed.
No change is needed in your end-user MongoDB or ORM/ODM code. The upgrade is as simple as update your mORMot 2 source, then recompile.