It happens on a private server as well, with only 2 people total and both already logged in. Suddenly both of us are having problems with certain things (falling through builds, building hammer stops working, etc). It does seem like the collision-issue happens at a particular base and, once triggered, affects everyone. Seems more like the build itself rather than login from our experience.

There’s a trick we’ve been using to get the game to recognize us when we fall through foundations (a particular modded piece that probably coincidentally works: we path through it and then, once through, game goes “oh wait, you’re not supposed to be able to do that” and suddenly collision works properly again) but otherwise we just relog and seem fine, until it happens again.