Use a train that you have seen to open the doors when you press T in some scenario on some route. (To rule out something bad with the train.)
The problem I am experiencing is in a test scenario starting at Trawsfynydd (doors don't open), stopping at Maentwrog Road (doors open and passengers load/unload), Festiniog (doors don't open), Manod (doors don't open) and terminating at Blaenau (doors open and passengers load/unload). The train is a pannier and two B-set coaches and obviously works fine as the doors operate a two stations. All of the above stations except for Blaenau use Exeter-Kingswear (EK) platforms with the separate INV platform for spawning passengers. Passengers spawn at all 4 stations. The platform at Blaenau is ex Woodhead (WH) and includes passengers without the use of an INV platform.
Make the platform marker really long, to be 100% sure that the whole of the train certainly is contained it it when stopping.
The platform markers run the full length of all platforms.
Ditch the stop-at instruction. It might eat up the mission step "get to X" and then the passenger instruction does not see you arriving at X as you are already there.
Done - no change.
Be sure that your Train Type in Scenario Editor is passenger train.
Train type is stopping passenger.
.....build a little test route ......
Done that. I built a simple single track 2 miles long, no signals or points with a EK pairing (platform and INV platform) at the start and the middle and a WH platform at the end. All complied with the conditions outlined in your post Andis. I then created a 3 stop scenario.
All platforms were populated. At the first stop (EK platform) the doors opened and boarding etc commenced. At the next stop (EK platform) the doors wouldn't open. At the final stop (WH platform) the doors opened and boarding etc commenced.
This was as simple a route as it could possibly be, no points or crossings, no signals, straight plain track with few joints and no joints in the platforms, long platforms with full length markers, fully populated along their entire length. What could possibly be wrong with the route or the train. It seems to me that there is some inherent unreliability in the EK INV platforms.
The next step is to delete all EK platforms and replace them with WH versions.
Groan!
Paul