I am seeing this same error, also on SDK 6.2.4, and it first appeared for me today as well. I will add that I am only getting this error with a trading client initialized using my LIVE secret/key. I do NOT get the error with a trading client connected to my PAPER account.
Here are the error details I received:
Newtonsoft.Json.JsonSerializationException: ‘Error converting value “ipo” to type ‘Alpaca.Markets.AssetAttributes’. Path ‘[1457].attributes[1]’, line 1, position 461914.’
Inner Exception
ArgumentException: Requested value ‘ipo’ was not found.
Edit: Just noticed your link with the same error details. 
The recent addition of an ipo asset status has impacted a couple of the SDKs. Working to update them to accommodate this change but no ETA.
@pflagler A new version of C# SDK has just been released which fixes the ipo attribute. If you can, let us know if there are any issue.
I downloaded the lastest version (6.2.5) and the issue is resolved. Thanks, Dan!