This has nothing to do with Isadora, but I'm simply posting this here for the good of the internet since I just lost 12 hours of my life to this stupid problem.

Mainly, there are a lot of articles out there about problems with file sharing on Windows 10 v1803. This all turned out to be a red herring and led me in the wrong direction. Things like turning on SMB 1.0 support, changing various settings in "Advanced sharing settings", and about one million more suggested solutions. (This article hits nearly all of these suggestions, but is perhaps the most confusingly written Microsoft article I've ever seen.)

But the problem had nothing to do with windows. Instead it was because we updated our FrtizBox 7590 and it changed a setting behind our back.

[SOLUTION]

I found the answer in this knowledge base article from FritzBox, specifically the step labeled "Allowing wireless devices to communicate."

Once I enabled the "The active wireless devices displayed below may communicate with each other" checkbox, my file sharing was working again.

I hope that saves someone else some time.

Over and Out,
Mark