Hi and welcome to JustAnswer,
Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, can you please consider/try leaving it plugged in even for a very long extended period of time?
Yes please; W OPE would be indicative of a Writing OPEration. Unplugging it and then plugging it back in would cause the upgrade/update saving process to start all over again.
Please post back results / observations of the above once tried.
Hi again and appreciate the post back. Sorry to hear the lack of development.
This would be indicative of a failed write process. The only option left then is to manually "flash" the firmware into the EEPROM of the SA-BTT775. This would require the use of a specialized jig and therefore must be done by an authorized Panasonic Service Center. Please contact Panasonic Australia 132 600 and have the below information ready.
In the mean time, keep the unit still plug in.
I realize that the above information may perhaps be not what was expected. Unfortunately, there is no quick-n-easy fix and neither is there a workaround; sorry for having to be the bearer of bad news.
Appreciate your kind understanding.
And again, sorry for having to be the bearer of bad news.