This happens on a clean installation of HIB#3, Build#2 (user hidden folder purged).
After taking the "harder" path (worm hunting mission which rewards you with grim mode) I reach chapter 1-IV ("The Council"). This brings me to the familiar round, lens-like window... but not any further. My vehicle keeps floating in the air inside the decorated window, and I do not even reach the full mission screen.
This problem seems to be unrelated to bug #5143 (https://bugzilla.icculus.org/show_bug.cgi?id=5143). The mission does not fail upon starting; it fails even to start.
Needless to say, this means I cannot progress further in the game.
I had the same problem, persistent with a new character and upon reinstalling the game (build 3 on Linux).
The NPC "zepelin" seems to crash to the right of the screen. All I see is the yellow arrow falling down and sometimes coins flying my way.
Created attachment 2939 [details] Unplayable saved game
Created attachment 2940 [details] Unplayable saved game profile