Have a look at this thread on Merlin's forum - Seems to be a known issue.... Doesn't bode well alas... https://www.snbforums.com/threads/wont-update-gt-ax6000-to-any-firmware.90784/ 

I known what you mean - I've become quite adept at clearing out the certificate store... Definitely a f/w bug - seen this now on two GT-BE98s.  Other Asus routers don't have this issue (GT-AX6000, RT-AX88U, etc. on stock and merlin f/w).

I have the GT-BE98 (as based in the UK).  One thing I've noticed is that SSL certificate is no longer valid after rebooting (or power cycling) the GT-BE98 router. You get error message: Your connection isn't private - net::ERR_CERT_AUTHORITY_INVALID....