Jonas Brothers -- Mystery Sober Coach Resurfaces

The famous sober coach who was tagging along with Joe Jonas last week fueling rumors that possible substance abuse may have caused the band to teeter on the edge of breaking up resurfaced last night with the boys.

TMZ broke the story ... Mike Bayer -- the CEO and founder of Cast Recovery Services -- left Chateau Marmont in West Hollywood with Kevin Jonas, after Bayer met with the entire band.

As we reported, Bayer is known for dealing with "crisis situations," and it appears the band is in crisis after cryptically cancelling 19 concerts a week ago.

We got Kevin at LAX earlier in the day and he was mum on everything.