There should be a copper or aluminium washer between the tensioner and the head.
Did you try to turn the crank by hand a little (taking the slack out of the chain)?
Maybe someone forgot or used a thinner washer between the belt tensioning lever and the timing case cover, causing the bolt of the belt tensioning lever to protrude to far inside/getting into contact with the chain tensioning rail?
Not sure about the 603, but could happen on my 662 (korean 602).