OK, I admit this error tripped me off.
If it were a file I created, I would have simply moved things around to reduce the filename length. But the error was coming from Visual Studio, specifically, from C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\15.0\Bin\Microsoft.Common.CurrentVersion.targets.
And, sgen: what's that?
It turns out sgen is just an XML serializer generator - something I clearly don't use or need. The .targets file above contained a task referencing Sgen.