A video making the rounds online shows the singer serenading a crowd at the Stockbridge Amphitheater a couple days ago ... when a security guard attendant appears to whisper something in his ear while he's in the middle of performing.

Tyrese keeps singing for a bit before abruptly jumping down from the stage and booking it up the side steps towards the exit, telling the crowd he loves them (and continuing to sing!)

Sources with direct knowledge tell TMZ ... Tyrese had 3 songs left when he got wind a process server was at the venue to serve him legal papers pertaining to a lawsuit brought by a man named Bryan Barber, who's suing Tyrese for defamation and libel.

Back in October ... Barber filed a $10 million suit against Tyrese -- claiming he bad-mouthed him on "The Breakfast Club" over footage Tyrese claimed the filmmaker was holding hostage ... something Barber felt damaged his rep, and which he's now trying to take him to court over.

Tyrese actually acknowledged the show on his Instagram -- but he didn't address the fact he pulled the plug early ... instead, he just mentioned the fact this show was dedicated to his late mother, Priscilla, whom he shouted out for Mother's Day.

He added, "Special day, two hours before the show I was going to cancel because my heart was so heavy. I just didn’t think I could get through it……. I all the genuine love and energy and the audience and just want to thank you guys for showing up tonight Bet you’re my brother from another mother you are incredible!"

Unclear if this is meant to somewhat explain him ending things the way he did -- but in any case ... Tyrese seems to remember the show fondly, even though we know why he bailed.

When this clip first started to go around -- with the notion Tyrese was trying to duck a process server -- some suggested it was all over another lawsuit he's facing from his ex-wife.

Remember ... she's also suing him for defamation -- but as it turns out, this ain't that.