I just (almost) finished our upgrade to Sitecore 9 (from 8-something) and completed the XConnect setup (just SIF as you would with a normal install). The installation (on a dev server environment) went fine, everything seemed to be running but after finally enabling the XDB on the (upgraded) site and testing it we get lots of errors in the logs (of both xconnect and website) saying:
[Error] An error occurred while communicating with the SQL Server System.Data.SqlClient.SqlException (0x80131904): Could not obtain information about Windows NT group/user '..\..', error code 0x5.
Which is probably true, as I don't want my sql database to know about my domain user. I see the logs with my account as well as with one of my colleagues. Weird thing is that I have been on the site after enabling xdb, but my colleague hasn't. He did visit the site a few days ago, but xdb was still disabled then.
I think I'm missing something - probably something silly.. but can't figure out what. I did check the connectionstrings obviously, and they are ok (using a sql user and pwd).