Well that is the second crank I shatter using a fixed rear wheel.

First one was a ISIS race face, and not only did the BB gave up, the crank holes for the bolts were all ovalised.

The second one is a XT, and when I was riding last sunday I ended up with only one bolt maintaining the ring to the crank.

What am I doing wrong?

Sounds like the chainline isn’t straight (wheel not straight in dropouts / dropouts bent?), or the chain tension is too tight. This could put uneven pressure on the chainring.

Cheers, I’ll try with the crank that comes with the bike, a smaller ring ,shorter arm and 5 bolt and see what happens

too strong – stop pushing the pedals sooo hard 😉

I shed chainring bolts more frequently when riding fixed, but I normally notice before I’m down to just one! I cracked a Middleburn spider by riding with only 4 out of 5 bolts fitted. My advice is to use threadlock and check regularly!

I agree with Pixelmix too – don’t get too anal about keeping the chain super-tight, so long as your chainline’s good.

You are using single ring specific chain ring bolts aren’t you?

ISIS bottom brackets are well known for only lasting a couple of rides.

Possible chain is too tight. That will arse the bb
But do use the cromo bolts we sell at charliethebikemonger.com Fixed is no place for alloy cheese.
Isis sucks. I go square taper on White industries cranks.

What am I doing wrong?

Nothing. Welcome to my world.