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When you enable FIPS and have an SSRS instance (or instances,) …"haha, good one, from SSRS 2016 and higher you can't have more than 1 instance because Micrsoft decided to screw us over.
I did not know this, thank you for the heads up. My work is currently on SQL2014, and my home lab I tend to keep at the same version for testing purposes. I'm rather surprised this was done, it's certainly going to make my next migration a bit more interesting (I've currently got one server with two instances of SSRS in production, and a QA server with four instances...)
They also changed the interfaces for custom security, so if you are using anything other than windows authentication you're going to have to change things there too. Basically HttpContext.Current always returns null in SSRS 2016 and higher, another case of grief caused by MS.