We are currently on 8.0 and upgrading to 8.3.  We did not use the web platform at all prior to now.  The windows client is working now without any problems, except for an occasional missing field.  I managed to finally get the web platform built and deployed, and I can get the login to come up, but I am unable to log in.  If I log in as Admin, it goes to a blank page.  If I log in as a different user, I get an error page with the following:

Value cannot be null.

Parameter name: key.

Saleslogix Error Id: SLX754EB177C72BA79E

URL: http://localhost:3333/SlxClient/Default.aspx

Exception type: System.ArgumentNullException

Source: System.Collections.Generic.Dictionary`2, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089

Note:  I have installed the SNC, web core, and web model updates release on 5/18/16, but still having the same problem.  

Any ideas?

Check if the APPID is NULL within the SYSTEMINFO table? If it is then you have missed a step within the upgrade process at some point. You should be able to populate the APPID column with: QSYST0000000 which is taken from the eval database. However, ensure the APPID in the APPIDMAPPING table matches the APPID in the SYSTEMINFO table.