In version 2.18 (and before) we used a property attribute to define a custom dictionary serializer as below:

[BsonSerializer(typeof(CustomDictionarySerializer<Dictionary<AccountConfigType, string>,  EnumStringSerializer<AccountConfigType>, CustomObjectSerializer<string>>))]

But after upgrading to 2.19,  we would get the error:

System.TypeInitializationException : The type initializer for 'i3vDB.Collections.Account' threw an exception.
  ----> System.ArgumentException : Value type of serializer is System.Collections.Generic.Dictionary`2[[Lib.Shared.Enum.AccountConfigType, Lib.Shared, Version=2.1.4.1, Culture=neutral, PublicKeyToken=null],[System.String, System.Private.CoreLib, Version=7.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]] and does not match member type System.Collections.Generic.Dictionary`2[[Lib.Shared.Enum.AccountConfigType, "Lib.Shared ", Version=2.1.4.1, Culture=neutral, PublicKeyToken=null],[System.String, System.Private.CoreLib, Version=7.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]]. (Parameter 'serializer')

Removing the property attribute, and instead using the below fixes the issue.

BsonSerializer.RegisterGenericSerializerDefinition(typeof(Dictionary<AccountConfigType, string>),  typeof(CustomDictionarySerializer<Dictionary<AccountConfigType, string>,           EnumStringSerializer<AccountConfigType>, CustomObjectSerializer<string>>));

I should also note that if you have both the property attribute and the explicit registration, it throws the same exception.    I did not test any other custom serializers as we didn't have any others specified by property attribute.

I don't know if it's significant as it could just be caused by the exception output, but the TYPE X doesn't match TYPE X message only differs in Lib.Shared.   The second instance is in quotes and has a space at the end.   They are otherwise identical.