This issue is known to the developers and they're working on it apparently.
From a small amount of testing, the issue seems to be that you can collide with multiplayer cars situated in grid slots 0 and 1 without obtaining any damage or feeling any contact even if you T-bone them.
Grids slots 0 & 1 can obtain damage, and will feel the contact.
In MP cars in slots 0 and 1 can go through each other. But anyone who collides with a car in slot 0 or 1 can feel the hit, though cars in slots 0 and 1 cannot.
To fix this-- since apparently the devs aren't interested in a fix-- you'll need to set two dummy drivers in slots 0 in 1. Just put the regular stuff in the entry list and then a fake GUID.
As far as open servers go, I don't think there's a way to get around this.
Major MP issue, as far as I see it. But it's been three weeks since AC has known about it. I am still of the opinion that this issue will never be fixed (though I hope I'm wrong), because they don't know how to fix it.