Version 0.12.14 is out now and has a fix for a multiplayer loading bug that was introduced in version 0.12.12.
We apologise for the several days multiplayer hasn't been playable.
For the full information on the larger 0.12.13 update that just came out which includes the SUPERHOT modifier for Workshop levels, check out the update post here:
https://steamcommunity.com/games/337320/announcements/detail/2931240984131812153
Changelist
Version 0.12.14
- Fixed Multiplayer not working