Likely related issue. 2.0.8 Stable has been our playthru for the last few days, until tonight when I attempted to login (while others were playing) only to get that error message upon login. As others exited the game to verify the problem, they had the same issue. I took the server down for ...

Legit. Thank you for the reply.

Are there any likely ramifications I should be aware of with the following test case:

Update to 16.14, check.
Then downgrade to 16.2, check.
Then incrementally go up until it breaks. Downgrade one lower, recheck.

My hope was to keep the map the same, in order to ...

Both player and server are in the Midwest US, one state apart.

I'd also note that some of the other players (including the one who tried to host a test game from his house for troubleshooting) live in the same general area (within a county or two of each other).

Player with problem also has no ...

So, this may or may not be a bug. I'll admit that right off. I run a private, dedicated physical server for several friends of mine. Server has been running fine, no issues, no other changes (Other than Factorio version files over the last week) or other games running at the same time. Server has ...