Cannot upgrade firmware on my USG 20W

Hi 

I'm trying to upgrade from 3.30(BDR.5) but it always fails with the error code : 42008 "Firmware not compatible"

I have tried with ZyWALL USG 20_3.30(BDQ.9)C0 from your download site - but also version BDQ.8 & BDQ.7 with same error

Currently the USG 20W reboots every 20-30 minutes with a app watch dog error - and cpu utilizing to 100% and reboots

Other recommend to move away from current firmware - as this is then fixed in later versions. 

Any help would be really appreciated 

Thanks 
Sonny