I'm also having this issue with XX530v V2 latest firmware (0.8.0). I've just bought an XX530v V2 from an online store and the first thing I did was upgrading the firmware with the one I downloaded from TP-Link support site. I also can't access the superadmin config. Tried resenting the router, tried Chrome and Edge on Windows and Chrome, Chromium and Firefox on Linux both normal and private modes always getting the "406 Not Acceptable" response. Also tried HTTPS by downloading the certificate from the router and installing on both OS and  browser wich works for secure access in normal mode but still get "406 Not Acceptable" on superadmin.

  @TomasChott Thank you, that worked. But it has to be the first thing done after resseting. If you do any other setting before that, the superadmin gets blocked. I usually set LAN IP to what I use in my network so I had to reset again.

  @Marcio_Miranda Yes, this is the basic rule, first set the SuperAdmin password via /superadmin, otherwise SA will not be activated and it is not possible to activate it.