Does the board you are attaching the vent to have weather stripping on it too?
When the problem is all of a sudden it hates me, I get the feeling, it normally is a build build up or environment change. For example, the wood board can warp with rain and humidity. It can also change due to heat and cold. These changes can introduce new holes where the smoke comes back into the house.
Do you have any screen material on the outside of the hose? This might have been put on to prevent insects from coming into the Glowforge or house via the exhaust line. Before I had even posted my first smoke filling room post, that was actually my problem. When I posted about it here the problem seemed to have been more caused by the onboard fan and other inline fan not having enough power any longer to remove the smoke properly.
Looks like you have the same external inline fan that I have. Have you taken the center fan piece out and cleaned it? Also, I have you gone outside and felt the air flow? Not sure if you can do this last one.
During my troubleshooting I had long running cut processes going on with draftboard to produce massive amounts of smoke and I slowly inspected the entire line process to see where things might be going bad.
Finally, a couple of tips. Go get a Filtrete 2500 MPR filter from a big box store. Make sure it is 20x20 and get yourself a box fan to tape it to the front of. This will greatly help in removing the smoke from the air between test runs as you figure out what is wrong.
Because mine is in the basement I also installed that filter in my AC as its filter. When the AC is on, the air intake was pulling the air from the basement and circulating it through the house. With the new filter, the AC now helps to clear the room of any odors that are normally lingering. This last point is really about when things get fixed.