The spectators were evacuated after the match was brought to a halt in the 57th minute, with the visitors leading 1-0 to trail 3-2 on aggregate.
The referee and a delegation from UEFA discussed how to proceed before the match was called off, AZ announced on their official Twitter account.
The remainder of the game will now be played on Friday at 11am local time, before the draw for the Europa League group stage.
The first signs of a problem came when the floodlights failed, with the apparent cause a fire around the roof of the stadium.
The crowd was asked to leave, while the players and coaching staff remained on the pitch before they too were told to leave the arena and the match was called off.
AZ tweeted a picture of the stadium with smoke clearly visible rising from it.
The Greek visitors will restart the match against their 10-man opponents looking to complete a second-leg comeback.
AZ, who led 3-1 from the away leg, were reduced to 10 men after only two minutes when Markus Henriksen was sent off.
Dimitrios Papadopoulos, having missed a first-half penalty, put the visitors in front in the 53rd minute, but they need another two goals in the final 33 minutes on Friday.