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Demi Lovato 's ex-fiancé Max Ehrich was put under another psychiatric hold after fans grew concerned, again, about his alarming behavior online over the weekend ... TMZ has learned.

Max tells TMZ ... he was taken by ambulance to a mental health facility in southeast Florida on Saturday after his Instagram followers raised red flags to police over a string of provocative photos and erratic posts on his social media. He says some of his fans feared he was suffering a nervous breakdown, and they alerted authorities. St. Lucie Sheriff's Office

We're told cops responded to his parents' home, met with Max, and determined he needed further evaluation ... so he was transported by ambulance for psychiatric assessment under Florida's Baker Act. He remained at the facility for 72 ho

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