I've had to perform an in-place upgrade to a Windows 2008 R2 server -> 2012 R2 -> 2016 for reasons out of my control. The end result is actually quite stable from what I can see/tell but have a few niggles I'm struggling to find a solution to.
The main one being with SSRS reports. Since the OS upgrade, browsing to reports hosted by SSRS on a dedicated server, with a date period field in them, will no longer load in an iFrame within a web application hosted on the upgraded server, however, works fine if browsing directly to the SSRS report outside of an iFrame. Also if I change it to a text field it'll work fine, so it's obviously got a issue with the date period field within an iFrame hosted by the upgraded OS server specifically.
There's nothing getting logged in "D:\Program Files\Microsoft SQL Server\MSRS13.SQL_BI\Reporting Services\LogFiles\ReportServerService__xxxxx.log" on the SSRS server when running this in an iFrame so doesn't look like it's making any calls at all. This is opposed to directly browsing to the report in the same browser, where it works as expected.
If I browse to a different environment that's running the same config but still running 2008 R2 servers, it works in the iFrame fine so is definitely something to do with the newer OS.