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The guy who played "Drop Dead Fred" in the 90s cult classic film passed away at home in England this morning ... and, so far, the cause of death is a mystery to his family.

Rik Mayall -- a huge comedic star in the U.K. -- was at home with family members when he died this morning ... but his rep says they're all puzzled about why it happened.

We're told the actor -- who was only 56 -- was not ill ... but his rep added they don't suspect foul play.

"Drop Dead Fred" was bashed by critics, but still became a financial success. Mayall was never able to score a second hit in the U.S. -- even though he's a big deal across the pond. He starred in several successful sitcoms there.