You won't believe it.
First of all, the Hebrew version of the story on the same site also mentioned propellers.
However, in an Israeli aviation forum it was said that the pilots were not sure that the flaps were up after take off so they returned to the airport. It was not even an engine problem.
El-Al stretched its fleet to the maximum in the past 4 weeks for several reasons. Now they have some time to fix problems before the summer season.
Long live the B747