Multiple law enforcement agencies tell TMZ ... Hart -- real name Michael Hartman -- has been accused of assaulting a person at a Mickey Hart Band show on Sept. 8 at Terrapin Hill Farm in Harrodsburg, Kentucky.
Hart played with the Dead between 1967 and 1995 ... the year Jerry Garcia passed away.
The details surrounding the alleged attack are unclear ... but the alleged victim filed a police report following the incident and initiated assault charges.
We're told it's standard practice in Kentucky to issue a warrant as soon as an alleged victim decides he or she wants to press charges.
Mickey's camp tells us they're looking in to the situation.