InvalidOperationException: Collection was modified in GraphBinary serialization

This was first reported for JanusGraph.Net: https://github.com/JanusGraph/janusgraph-dotnet/issues/140

but the issue is most likely in Gremlin.Net.

System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
  at System.Collections.Generic.Dictionary2.KeyCollection.Enumerator.MoveNext()  
  at Gremlin.Net.Structure.IO.GraphBinary.TypeSerializerRegistry.GetSerializerFor(Type valueType)
  at Gremlin.Net.Structure.IO.GraphBinary.GraphBinaryWriter.WriteAsync(Object value, Stream stream, CancellationToken cancellationToken)
[...]

The problem is most likely that the driver iterates over all supported types in a foreach loop to find a type to which the type to be serialized can be assigned which means that the same serializer can be used. Then this serializer is registered for the new type which modifies the collection the foreach is iterated on, leading to this error.