This post may help:
I figured it out. This JIRA ticket has the details.
Effectively, we’ve made a distinction between connecting to a
standalone server and connecting directly to a replica set member,
where the latter is relatively uncommon. Unfortunately, MongoLab’s
Single-Node settings are actually a single node replica set and this
causes us to not trust it. You can fix this by appending
?connect=replicaSetto your connection string. It will force the
driver to move into replica set mode and all will work.We are going to re-consider CSHARP-1160 in light of this. Thanks so
much for reporting and let me know if appending?connect=replicaSetto
your connection string doesn’t work.