It turns out the second time was not the charm for KSI and Dillon Danis to meet in the boxing ring -- their big fight slated for next week has been pushed back ... but surprisingly, it's not the latter's fault.

KSI broke the news to his fans Friday morning ... saying his body has given up on him as he deals with an illness.

"I'm coughing up greenish-yellow s*** all the time," he said. "My sinuses are hurting to the point where I need ibuprofen every day. The tension headaches that I'm getting are more painful than you can imagine."

"Basically, I'm f***ed. I've been ill for almost three weeks, and it just has not got better."

He hopes to move the fight to another day ... but there's no word yet on when that will happen.

The YouTuber-turned-boxer concluded his statement by apologizing to DAZN, Misfits, The Sidemen and the fans for letting them down.

They were initially scheduled to throw down in January 2023 ... but Danis pulled out -- with KSI's team claiming Dillon was "unprepared" and struggling with his weight.

The new bout was set to go down on March 29 in Manchester.

As you would expect, it didn't take Danis long to respond. Calling his opponent a "p****" and joking, "I remember my first cold."