XR400 gearing/gearbox problem

[Gotham]

Hello XR gurus!

Now this is second time when this happened with my bike. Chain runs off the drive sprocket so it misaligns with it and then the chain tension goes very tight and blocks the engine. When it's correctly on sprocket chain free play was 45mm.
Both times the drive sprocket was surrounded with dirt. As the drive sprocket is not bolted to drive shaft there is axial movement, so could the dirt cause this misalignment with the chain or what it could be. Chain slider? Something else?
First time it happened after a longer jump when I didn't accelerate on landing.
Of course I crashed and the front chain/sprocket guard bolts were also bent.
Gearing was 15/45.
Second time it happened on this weekend 2-days enduro race. After 60km I was crossing a simple mud hole with first gear (usually on second) on full throttle and helping with my feet. When I was out of it I realized that the back wheel didn't rotate and I had just pushed it out. The chain was tight as rock and didn't move. When I pushed bike backward the chain was on sprocket again and and tension was OK. Gearing 14/45. But now I found really unpleasant surprise. Engine was running fine but when I switched first and second gear in nothing happened, the bike didn't start to move. Tried many times. Anybody know or has experienced similar problems or know where to look the problem, obviously in gearbox but some hints would be good before I open it. I think this joke will be costly.
BTW bike has done 2050km-s.

Cheers

[Gotham]

Little update!
I haven't opened her yet but right now I think the problem is with clutch. When I rotate rear wheel and switch gears (they operate normally) there is difference when it's on neutral or in any other gear. In neutral the resistance is smaller than in other gears. So I guess I burned the clutch or harmed it any other way when I was in full throttle and the chain stuck. I remember there was a moment when the bike wanted to strangle (I was surprised a little) but I automatically helped it with clutch and opened full throttle (still on first gear). Try to check it out on weekend.
But still there is question how to avoid that it will never happen again, i.e. the chain runs halfway off the sprocket? I doubt that 5mm smaller chain slack would have helped as 40mm slack is written to swingarm.
Has anybody else experienced this or have any ideas?

[AIRWOLF]

Is the chain guide in good condition ? (the one under the swingarn that guides the chain onto the sprocket).

The burnt out clutch can happen anytime, . . especially working it hard like that, . . . probably just the straw that broke the camels back.

[Gotham]

Is the chain guide in good condition ? (the one under the swingarn that guides the chain onto the sprocket).

The burnt out clutch can happen anytime, . . especially working it hard like that, . . . probably just the straw that broke the camels back.

Thanks!
I have suspected the guide as the chain do not or haven't ever run truly on it, i.e. the chain wore new grooves on it. But I was too lazy to look it closer and fix it. But I think that more obvious reason is chain tension even though I said other things before. Manual says 35-45mm I have always thought that looser is better but now I don't. If some dirt pushes drive sprocket to one side and chain to other, the probability for the chain to run off the sprocket is bigger with looser chain than tighter.

[AIRWOLF]

I run my XR without the sprocket cover/gaurd so the dirt can clear easier. you do need some sort of case saver though.

[Gotham]

I run my XR without the sprocket cover/gaurd so the dirt can clear easier. you do need some sort of case saver though.

Yes, I thought that also and now I think that is what I'll do.