Eurobracing break

[Jason McK]

OK so of course while I'm away in Ontario, I receive a frantic call from my wife who tells me a large chunk of the eurobracing has just broken off my tank.
It's a 3 inch triangle and then a large crack runs from the outer edge along the center of the bracing for about 3 feet.
I'm home now and have checked everything.
the tank is not bowing out at all. and I don't really understand what has happened. This is actually the second crack. The first one was on the back of the tanks eurobrace and has gone unrepaired for about 1 1/2 years now. But that one had no glass removed.
My eurobrace is about 3 inches wide and 5/8 thick. I put a level on all 4 sides of the tank and all sides are level. Also the bracing on either side of the missing triangle of glass remains in line.
The manufacture says it might be heat related but I just don't feel any heat of the bracing even after the the MH have been on for hours.

I need some suggestions, a 400G tank is not something I can pull down while I get a new tank.
the manufacture has suggested siliconing a 1/2 piece of glass over the crack. that would work great but all the glass shops I've called said cutting a 1/2 inch think piece of glass is very difficult and will be very expensive.

Any thoughts would be great

Thanks
J