It already happened before in about '91. The entire fleet was first grounded, (after a very nasty incident at Binbrook) then the restriction was partially lifted to allow cadets to go solo - with an instructor along 'just in case'!! It was called a 'ghost solo'.
I think it was caused by the use of MOGAS, combined with long periods of carb heat causing vaporistion in the fuel lines, but maybe someone else can elucidate.
(NB It does strike me as strange that a powered glider losing its engine can be considered dangerous)