Mongo write error: crash on new file failure

When trying to insert into a collection, I encountered Mongo write error (code 12520, message "new file failed"). I can insert into other collections in the same database without issue and there is disk space. I set the ulimits as recommended in the documentation and followed the instructions here , it cat /proc/<mongod_pid>/limits

shows that higher limits have been applied. The problem first appeared with documents about 7.5 m in size, and after emptying the collection, it reappeared in 7653926 documents.

Does anyone know what the problem might be or where to start looking?