Garmin Forerunner 645 causing a rash with stock strap

nope, wearing mine for 4 weeks now, nearly 24/7 and have no problems. It is worth trying to loosen the strap a bit close to the point your HR sensor gets problems with accurate readings.

I have personally had a similar issue at the beginning of using the watch, but it seemed to "sort itself out".
Washing the strap after each workout and sometimes switching which arm you have the watch on will help out the skin to breathe.

However, I would recommend getting another type of strap that is a little bit more"breathable".
I've heard from a family member that changing the strap got rid of the "red rash issue" he had with the Forerunner 935.

Yes, I definitely have/had this too.  As Run4Fun said, washing it after activities has helped me.  Loosening it slightly out of activities, and also switching wrist frequently, generally resolves this for me.  I wear my watch overnight, for the sleep information.  I'm sure removing it overnight would help, if you don't need that info.

I had it initially but it sorted itself out more or less. Then I switched the strap to one made out of cloth and no problems at all.

I do get this sometimes. I try to take the watch off for a bit, use some additional lotion for a couple hours... then put it back on. also wearing it just a bit looser (but still good for registering the HR) seems to help.

Had mine since christmas and just switched to a leather band last week, but been using the one it came with since Christmas and no issues.

The band isn't the problem for me.. it's the 3 green dots that are used for the hr.. that's where i get a bit of redness and skin peeling. 

Oh dang interesting no issues for me. That sucks sorry to hear that.