Hello!
I had problem on  Rajd Baltic Cup (Polish Rally Championship) at this week.
Car -  Subaru Impreza N14 N4 (NGP_GRN_1), tyres - snow.
The behavior of the car was abnormal: in the snow stages, the tyres friction was normal,
but at the Monte-Carlo stages it was practically absent. The car was sliping and rolling at
a speed of less than 30 km/h. In Alone mode with same parameters behavior was normal.
What can be reason for this difference in the Alone and the Championship modes?

Hi, it's normal that your car was slipping because your tyres are worn a lot of, in NGP mod the wear of tyres is more important that in normal mod, you have certainly drive too fast in snow mod and after it's very difficult to drive car in MC mod, it's a inconviant of the NGP with snow tyres on the tarmac, sorry for you.

In Alone mode with NGP physics enabled the wear of tyres is not so important as in Championship mode?
Or this is cumulative effect over muttiple stages? But on snow stages tyres wearing must not be very essential.
In my case tyres friction was very different from Alone mode at the first MC stage!

Yes that it, in championship the wear of tyres and damage is cumulative in the same round.

I have exactly the same problem, the car gets absolutely undriveable after a few stages, up to the point where it is completely unrealistic, almost cartoonish.  

I know that there is tire wear, but some of us are more affected than others, and at least in my opinion this is some kind of bug in the "old" NGP plugin and not due to overdriving, since we are not using the most up to date version if im correct ?

During warm up everything is alright, as Gres alreay mentioned.

First stage with Snow Tires @ Monte Mod - No overdriving, reasonable good times

Second stage with Snow Tires @ Monte Mod - No overdriving, but noticeable less grip, times are going down a bit, still semi competitive.

Third stage with Snow Tires @ Monte Mod - Almost undriveable, the car does not want to brake or turn it at all, no matter what speed, only with aggressive hand brake maneuvers i am able to throw the car in a direction pretty much. No matter what gear (long gear ratio as well btw), absolutely zero traction in a 4WD WRC car, no matter how low the rpms or how high the gear might be, the car only turns in at walking speed.

Fourth stage with the now worn to sh** Snow Tires @ Snow Mod - Driveable again, but very bad turn in and bad traction, over 20 seconds slower than on fresh tires during warm up, on a relatively short track.

Fifth stage - Zero turn in, zero traction, crashed the car - Rage Quit  

I remember a Monte NGP Rally one or two years ago, at a certain point i reached a grip level where i pretty much had to drive walking speed, my time for a finished stage without incidents (but zero traction) was slower than the time given for being retired   - Absolutely impossible, in my opinion at least.

On an all dry rally i have zero issues at all, when i am forced to run "compromise" tires on a rally with mixed weather conditions, my tires will be blown to pieces after 2 stages.

I already compared replays, some people just dont have those tire wear issues at all, some do .....

Yes, it's seem so. And I see, that the times of some another drivers on MC stages with the same cars are normal.
But my times on MC stages exceed retired times, when on snow stages they are close to others!  
What's the reason? Am I right: NGP plugin is the same for all, it's installed with the RSRBR mod, and I couldn't upgrade it?