If you are receiving that error then you have the wrong firmware.
What makes you think your problem is firmware related? And just because a device allows a firmware update, that does not mean the manufacture will provide an update. The OSUB Plus 80 is 3+ years old since it’s release – well beyond any support window the majority (if not all) of Chinese manufactures provide. If the manufacture does not provide a FW update, or even the original FW, then I would be very hesitant trying some file that was just found on the internet. And not just for the risk of your device, but the risk of your computer as well.
How long have you owned the OSUB Plus? Might be time to cut your losses and purchase something newer.