It is also strange, that I cannot buy tickets / reservations on the MOB website for this connection  (e. g. November 6th, 14:02). The error message is to select another date...

On the Golden Pass Express (train category PE) the reservation is not compulsory, but it is possible.

To use the prestige class you need a first class ticket and on top a reservation for this class.

Depending from the day and time it is a good idea to buy one to have for sure a seat.

Thank you. So I do not need a reservation e. g. on the 14:02 Zweisimmen to 15:15 Montbovon train as it is a PE train.

When I have a first class interrail ticket, which class could I use then in the train without a reservation?

Any idea, why it is not possible to get a price / ticket on the MOB site for this train?
(is it already fully booked or is it not possible because it does not run the whole route to Montreux but is replaced by bus between Montreux and Montbovon?)

Do you think, now in November, trains are full as it is not high season?

All Golden Pass trains have optional or no reservations (never required). Prestige requires a supplement. With a 1st class Interrail pass (which is your ticket) you may sit in 1st or 2nd class.

The new gauge-changing Interlaken - Montreux services tend to be busy but I've still found a seat the 3 times I took them. For the others (xx:02 from Zweisimmen) you'll 100% be fine without a reservation, esp. on a November weekday. Don't worry! Btw the windowhangable carriage is less busy in my experience.

About tickets: it could be an issue on the MOB website. Always trust and rely on the SBB website. Tickets cannot sell out in Switzerland. :)