I've been mislead by this error more than once. After spending hours googling, updating nuget packages, version checking, then after sitting with a completely updated solution I re-realize a perfectly valid, simpler reason for the error.
If in a threaded enthronement (UI Dispatcher.Invoke for example), System.IO.FileNotFoundException is thrown if the thread manager dll (file) fails to return. So if your main UI thread A, calls the system thread manager dll B, and B calls your thread code C, but C throws for some unrelated reason (such as null Reference as in my case), then C does not return, B does not return, and A only blames B with FileNotFoundException for being lost...
Before going down the dll version path... Check closer to home and verify your thread code is not throwing.