I've had this happen before...
is the gazebo all one brush?
if not, that is good.
try deleting the offending brush and re-adding it.
It looks like a simple cube so re-adding shouldn't be a problem....
otherwise if brightcorners, or dirty shadows doesn't work I don't have a clue...
/EDIT also try repositioning it abit (Hourences fav thing to do

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