When using this form of c# code (simplified):

>> var collection = MongoDataConnection.GetCollection<T>(true);
>> IQueryable<T> qyb = collection.AsQueryable();
>> var cursor = qyb.Where( e=>e.Name.Contains(astring) ).AsQueryable();

The resulting mongo command is converted to:

>> collection.Find(

{Name:/astring/s}

)

which fails with Invalid flags supplied to Regex constructor 's'
This was also consuming 99% CPU on our servers. This problem has only just materialised.
When in shell I run :

>> collection.Find(

{Name:/astring/s}

)

I still get the error - if I convert this to

>> collection.Find({Name:{$regex:"astring",$options:"s"}})

it works fine..

–

This is a HUGE issue for us.