TAYLOR SWIFT is being sued by a Canadian ticket company for KEEPING the $2.5 million they paid her in advance . . . even though the concert was cancelled and she never performed. 

--This was the "Capital Hoedown" and it was supposed happen last August in Ottawa, Canada.

--Taylor is actually being sued for $1.8 million . . . because that's what the ticket company had to hand out in refunds.  So even if Taylor DID fork that over, she would still walk away with some $700,000.

--Taylor's people are sounding like weasels about it.  A rep told TMZ that their deal was not with the ticket company  . . . so she's not responsible for the refunds.  They also claim they have not yet seen the lawsuit.