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Indianapolis Colts QB Chad Kelly is being sued by Von Miller's personal cameraman -- who claims Chad socked him in the face while drunk and high at a party in 2018 ... breaking his nose in the process.

The man behind the suit is Jackson Belcher, a videographer who's been working for Von Miller for years.

Belcher claims he was hired to cover Von's 2018 Halloween party at The Gothic Theater in Denver on Oct. 22 ... where he claims Kelly (Miller's Broncos teammate at the time) was boozing and using "recreational drugs."

In his suit, Belcher claims he went up to the 2nd floor to retrieve a camera when he was "inexplicably grabbed" by Kelly ... who was "obviously intoxicated, enraged and highly aggressive."

Belcher says Kelly punched him "directly on the bridge of his nose, causing him to fall into the group of people, and then onto a railing."

Multiple security guards raced over to restrain Kelly, according to the suit, but he kept fighting ... punching, kicking and throwing elbows at the hired muscle.

Belcher says he suffered a broken nose in the attack which required 4 surgeries to fix -- along with a head injury, concussion and physical and emotional trauma.

Later that same night, Kelly was arrested for breaking into a random person's home while dressed in his Halloween costume. He later pled guilty to 2nd-degree criminal trespass.

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The Broncos cut Kelly -- but he was later picked up by the Indianapolis Colts, where he's still a backup QB.

Belcher is suing for assault, battery and more. He's demanding unspecified damages including the cost of his $14,000 camera which he claims was damaged in the attack.