Munich to Venice- error ''search an alternative route''

No-otherwise there would have been another message and it would all have gone on clear.

Likely there will agian be works somewhere along the route and this may mean parts of it-thats just 1 of the probs one has with these old-fashioned rails-cannot be done. In fact this train when it leaves München consists of 3 different parts-the fun thing is that cars from trains can be shunted from 1 to 2-the car changes trains, not you. You will have to leave that car/space in the mid of night, I am afraid, with logic thinking of what you present here.

OR it may mean that the direct cars are fully booked (NJ seems extremely populair this summer) and that hence you have to change cars/space due to that-at the change-over point where all these cars get shunted. Your seat 1 will be claimed by someone else from that point.

YOu did not clearly state what means you used to book: this should be OeBB-first use planner and then it will utmost clear show what the matter is.