In my 15+ years of working in this industry, I have never seen a biofalls or skimmer box crack or split, but I have only dealt with the better known brands which are constructed of heavy ABS. I have seen them bowed and severely buckled but still maintained their structural integrity.
A one (1) year warranty should have been a warning sign. I know of no product available for field use that will patch/repair a crack to a level that will resist the pressures that these units experience from the surrounding soil.
Rip it out and install a unit that is manufactured by one of the better known manufacturers. Remember, when you buy cheap, you get cheap!
"Everybody is ignorant, only on different subjects." - Will Rogers