Just "upgraded" from 5.0.7 to 5.1.0 - then unable to log in as the password appeared to have been changed    Wouldn't accept default password either so had to re-set.  Reverted back to 5.0.7 in case of other issues....

Had the same problem of password being rejected but got it sorted by clearing out the Chrome Browser cache, it then accepted my password and has been fine since.
Had to do the same with my 962c.

Neil..

But how does the browser cache affect a manual input of the (same!) password? I don't doubt your resolution of the problem, but what is the explanation?

edit - well, whatever the mechanism, upgrading from another PC (with browser cache cleared) has allowed me to login.  Never had to do that with previous router/AP upgrades though.

second edit - having succesfully logged in one time it's now not letting me login again    There's definitely a problem with this firmware as far as I'm concerned.

third edit - if I go to http://192.168.1.5/web/#/login  (which is what my 2927ax decides is the AP address in the "External Devices" list), I get a "500 internal server error" and can't log in.  If I go to http://192.168.1.5 I can login straight away.

I aslo note that the 805 dashboard thinks that it has 1 client on 2.4GHz even though nothing is connected.  The mesh config is not syncing to the 805, but I suspect that may be because I'm running an RC firmware on the 2927ax, so I shall defer judgement on that.