I don't do much Modifier & VBA anymore (mostly VS Tools for customizations), and have only had a handful of customers upgrade to GP 2015, which is probably why I haven't run into this error until today.
While testing an upgraded VBA customization on GP 2015 today, I received this error.
When I clicked on Debug, I was taken to the call to UserInfoGet.CreateADOConnection.
Thankfully there is a Community Forum post on this error, and the discussion indicates that this was an issue with GP 2015 RTM. A post on that thread indicates that GP 2015 version 14.00.0661 resolves the issue, and I see that I even posted to the thread confirming that the problem was resolved with 661.
But today the problem occurred for me while running 0661, and I just confirmed that my client is running 0661 and is receiving the error.
After upgrading to the October 2015 Hotfix (KB 3089874, 14.00.0817, post R2 update), the error went away and my VBA code and UserInfoGet calls work fine.
So it seems that somehow the problem can still occur with version 14.00.0661, but is resolved in a newer hotfix.