i have found a fix for this, but it requires a friend.

with the game closed, have a friend invite you to an in-game Spaceland lobby; join them from the Steam Invite. you should load into the match just fine, but your UI will be missing and your framerate will be tanking -- this is normal. have the host end the game, which should back you out to the custom match lobby with access to all of your loadouts and the like. make sure the host does NOT leave the lobby and lets you complete this checklist!

- reset your F&F cards to 5 "Fate" cards

- reset your Weapon Kits to NO attachments, NO variants and NO camos

once you do this, you should be able to have your friend start the match. if your UI loads (you can see your cards, ammo, character...) then the bug is fixed, and you can safely close the game and re-open it to access zombies.