Sounds like a scenery problem mate and nothing to do with the Aircraft. Did you install any addon scenery around the same time?
Re the multiplayer, there is a program you can run to host session, but tried it once and it was rubbish, the lag was terrible. Other than that, its Gamespy, which I must admit was pretty good before. We did a reinactment of the Dam Buster raids a few years back and it was fine for most of that. DB and I managed the whole flight... 4 or 5 hours if I remember correctly... actually, it may have been more. Most of that was at 200Ft!
... and the trick with the multiplayer sessions is to make sure everyone has the same aircraft installed otherwise everyone will appear to be flying the default 737 and airbuses etc.
I havent installed any scenary for a while now, and the notice only comes up when I go to select an aircraft, and is identified as being a problem with the HS748 in particular.
Not to worry, Ill live with it for now.
We have only just recently started using textures for the default 737, and the other guys I fly with all make sure they have the same, as you say if you havent got it on your files, it wont show. My Greek and Canadian friends were online last night having installed some textures, I had Easyjet, the Greek has Olympic, and the Canuke had Air Canada or something similar!
I am in the process of teaching them about the ILS and the vetor procedures using the gps. I am also trying to teach the Greek guy how to fly helicopters, now that IS fun ! LOL.
The Dambusters sounds like fun, must be tiring though, 4 or 5 hours manually flying!