We got on the train at CDG at around 6.30 pm today. (15.4.23) At Garde de Norde it stopped and everybody had to leave. Now I'm afraid something similar will happen when going back in tuesday morning. We plan on getting on at the Luxembourg station around 7am.
Checking on the RER B twitter feed, it looks like someone jumped onto tracks at Cité Universitaire station (about 5 stations south of Gare du Nord), resulting in a lengthy shutdown/cleanup situation. That's a few stops further south of Gare du Nord. I'm assuming they were terminating the RER B trains at surface level at Gare du Nord to reposition those trains to head back north towards CDG and allowing crews time to resolve that situation on Cité Universitaire.
You should be fine for Luxembourg back to Gare du Nord on Tuesday.