score:6
Wikipedia has a list of annulled elections and the 1993 election in Nigeria seems to fit your question reasonably well.
Another example seems to be Myanmar in 1990.
Afghanistan actually has a repeating pattern of Pashtu und Farsi-speaking politicians forming a government of national unity,then accusing each other of fraud after the next election, then forming another government of national unity. Not sure if this counts.
I originally had mentioned Mongolia, which had a national unity government from 2004 to 2006 or 2007 and again after 2008, with allegations of fraud after the 2008 election. However, there is no real way to call those who made the allegations in 2008 "incumbents". (FWIW, I think widespread election fraud would be considerably harder to pull off in Mongolia than in Afghanistan, because Afghanistan has much deeper political and ethnic differences between regions.)
I had suspected that election fraud was also cited in the decision to call off the second round of the Algerian parliamentary elections in 1992, but I did not manage to find any specifics. Maybe someone else will be more successful. I would expect that election irregularities are a common pretext in such scenarios (as in the Myanmar one cited above), but it is somewhat hard to track down the specific justifications and pretexts in each case (also as in the Burma example cited above).