I guess you forgot to connect TLP250 pin 5 only in your schematic?
The inductor is loaded with a certain amount of DC current according to the asymmetrical bridge (two-switch forward converter) topology. Are you sure that it isn't saturated?
With the shown transformer output voltage, the gate voltage might exceed absolute maximum rating of 20V. I think it's appropriate to place small TVS or Z-diodes parallel to GS.
Apart from these points, the reason for failure can't be neccesarily seen in the schematic. It may be a problem of circuit layout.