Intake only problems - cathedral ceiling

I have here a six year old SBS roof, nailed base w/ cold adhered cap sheet. 2/12 - The shingles were torn off, decking repaired as necessary, and mod bit put down. Chimney was sealed w/ roofing cement, other than this it's a solid installation.

Now the decking is rotting out from under the membrane. It VERY bad between three rafters for about 15-20', starting about 4' up from the eave. Then another similar issue near the upper corner of the roof, but not near as big.

Thing is, it's not leaking. No stains on the ceiling, no signs of leaks, roofing looks good. It does however have circular soffit vent's along the bottom. Could those have caused this plywood to complete disintegrate in six years? Any thoughts appreciated.

Good idea about the frost. We had just recently started getting frost again, hadn't thought of that yet!

We're covered up now though!

I couldn't find a source inside. Had a energy auditor come out w/ his infrared gun to see if we could pinpoint anything, but it was inconclusive.

Chimney is mudded, but no leaks on the inside around it and the plywood touching it is still solid.

OS, what do you think would be the result of blocking the intake and installing an epdm or similar roof? Creating the dreaded hotroof.

Thanks for the input OS. I've built several 2x roofs, they do work great :) But I've never seen this kind of damage in such short time. And it was usually distributed throughout the roof. I just want to make sure that COULD be the problem before I make plans to fix it.