Glad you hopefully have it figured out.
The only time this happend to me was with a 93 ZX-11 that I had just had an aftermarket exhaust (full system) and a jet kit installed. Trailered it to Daytona for bikeweek...rolled it off the trailer...rode into town...got caught in traffic...hot as all get out in Daytona and yep...coolent started bubbling out.
What my issue turned out to be is that my aftermarket header was bigger and was too close to my "plastic" fan
and yep...the fan blades melted.
The rest of the weekend was ok as long as I kept moving and kept the air circulating...no more downtown riding.
Got home...swithed to an aluminum fan and no more issues.
Live and learn.