Tori Spelling Cancels Appearance with Ex Brian Austin Green Due to Illness

Tori Spelling won't be letting it ride in Las Vegas this weekend ... revealing she's sick and has to cancel an appearance with her ex in Sin City.

The actress announced the news via Instagram Friday ... telling her fans she reluctantly couldn't host the '90s After Dark Party on Sunday with Brian Austin Green because she's dealing with a major illness.

Spelling tells her fans it hurts her to cancel her appearance ... but she needs to prioritize her health this holiday season -- and, she thanks fans for their love and understanding.

The '90s After Dark Party is held at Voltaire -- a popular nightclub in The Venetian on the Las Vegas Strip -- which is supposed to transport fans back in time to the 1990s.

Of course, Tori and Brian famously played a very popular couple on "Beverly Hills, 90210" back in the '90s ... and their courtship even leaked over into real life.

TS and BAG dated in the early 1990s ... and, despite the breakup, the two have maintained a good relationship over the years -- with Brian even joining Tori for an episode of her podcast last year.