I believe that the official reason for these aborted takeoffs noted above was the "thrust reverser message". I saw this in today's Wall Street Journal in the US:

Boeing 747 Fix Ordered

"Concerned about potentially hazardous takeoff problems affecting many of the largest Boeing 747 airliners, air-safety regulators have proposed mandatory fixes to ensure the jumbo jets will climb properly...The FAA and Boeing discovered that during takeoffs, sensors on equipment called thrust-reversers can prompt some leading-edge flaps to retract without pilot commands...The Agency's proposed directive, issued last week, covers nearly 100 747's flown by US carriers...more than 460 planes flown by foreign carriers likely will be covered by similar directives expected to be issued by air-safety regulators in Europe and elsewhere..."