COOP & mod.io not accessible

I'm kind of posting here as a last ditch effort. I spent 10+ hours trying to fix this over the past few days without any success. I played Snowrunner with some buddies back when the game originally came out. We were looking to hop into some coop this past week only to discover that I could not get coop to work on my PC. I select coop and choose my save slot and then the game gets stuck on "Checking permissions", then says its stuck in offline mode. Single-player works without issue. The in-game mod browser never shows a mod.io login prompt, and Documents\My Games\SnowRunner\base\Mods.modio stays empty (no authentication file is ever created). The log files in the logs folder are 0 bytes, so no useful help there. I did some googling, I checked the steam forums, this subreddit, and some of the other related forums and tried every fix recommended to me. I reached out to Saber through their online bug submission system and all I got was an email asking if I had any mods installed, despite not being able to connect to mod.io being the primary problem.

What I have tried:
Network: mod.io loads in a browser, nslookup and a direct HTTPS request to api.mod.io succeed; I tried default DNS and Google's DNS servers; I don't use a VPN; I use default Windows Firewall which I manually added inbound/outbound rules for all of Snowrunner's ports; I manually portforwarded all of Snowrunner's ports on my router to my PC, and when that failed I set the router to DMZ mode. I used Wireshark to see what was failing when the permission check was happening and as far as Wireshark is concerned the server handshake went through.
Install: verified files, full clean reinstall, and deleted the Documents\My Games\SnowRunner folder.
Steam data: turned off Steam Cloud and deleted Steam\userdata<ID>\1465360.
PC profile: tested with a fresh windows install on an otherwise formatted drive – all I had installed was windows, steam, and Snowrunner, and coop would not launch. Other: I ensured Steam status was set to Online, I tried launching the game from .exe as admin, I tried all the hack fixes you see recommended online – IPv6 off, Windows Time Service synced, Windows updated, double checked Documents folder is not in OneDrive, etc.

At this point I really just want to play with my buddies and maybe try some of these fun new mods. I'm curious if anyone has any recommendations for anything else to try.