Takeoff configuration warning question.
 

B735 rejected takeoff after TO configuration warning.
All lights indicating positions of LE devices on central and aft overhead panel were blanked after RW vacation.
Pilots confirmed of LE FLAPS EXT light illumination before takeoff.
What was the reason for LE FLAPS EXT light to extinguish during takeoff?
Thank you !

Where was the flap selector?

At flaps 5 position

There is not much information to work with here.

Did you check the FSEU? This illuminates the TRANSIT, EXT and FULL EXT lights and sends signals to the Config Warning.

Rgds
NSEU