Still plaguing the game since the s&box networking optimization update, unfortunately. Almost all players connecting will fail a few times until they get in, and most players don't try a second time. Player counts have really gone down.
I hope for Linux binaries soon, if this is a Wine issue after all. If you fail to connect, send logs to me so I can amass more repro info.
This reduction in players of course, makes it less motivating to work on the game. It really is unfortunate.
A bit of news since I wrote this initially; it might have been a completely different thing causing all this, I'm gonna need more people trying to join to confirm if they still get the error. If you do get disconnected, please send me your logs!