... the login doesn't work for me still (4/5/2021). EDIT: well, unless I change a browser default explained below.
Without the change...
What happens under Safari and Chrome is fairly identical. When I click on the "Sign In" button, I get a full-screen username/password authentication window. When I fill this out correctly and submit - the site refreshes with an empty form with the same window asking for my name/password. No errors - but no login happens either.
If I request the desktop site (rather than mobile) - then clicking on the "Sign In" button will flash the button then reloads the page - but doesn't ask me for any credentials.
This is all under iOS 14.4 and primarily I test under Safari (and just spot-check Chrome to see if it's any better -- it isn't).
Under iOS settings - there's a bunch of settings that one could fool with. There's the "regular" ones like "Block All Cookies" (which I have turned off) and "Prevent Cross-Site Tracking" (which I had turned ON - and now turned off for more testing). Guess what - now I can login.
Ok - maybe BM just has the magic settings.
1) iOS "Settings" icon that looks like two gears with a "Y" pattern of spokes connecting the inner to outer gear
2) On the left-hand column "Settings" find the 4th or 5th collection of icons that starts, for me, with "Passwords", "Mail" ... and then 8-10 icons down is "Safari". Touch "Safari"
3) Now in the right-hand column I see different headings. "Allow Safari to Access", "Search", "General", "Tabs", ... scroll down, "Privacy & Security" ... stop - it's here. The first option "Prevent Cross-Site Tracking" - I turned that OFF.
Now after proving credentials, the screen does change. I have to press the back button after login. It does work.
These changes didn’t seem to fix Chrome although I didn’t restart Chrome after the change to test. EDIT: restarted Chrome and it didn't help. Use Safari.
This is just my 3rd or so time up to bat with this problem and I didn't want to strike out. I thought about the fact that BM had this working and forced myself to start going through the browser settings. Before I gave up thinking about some fake differences between BM's network environment (like VPN access to Yamaha's network) - but I dismissed this crazy theory because I doubt he would VPN in all the time on the road for using the site - or even if that's a thing used at all.
I looked at the "millions" of settings under the experimental/advanced section of Safari and really didn't want the problem to be there. So I just started with what made sense in the "standard" settings using educated guesses and the first guess provided the answer.
I suspect the problem is with one domain (like yamaha.com) handling the cookies for the login/password process and then using those cookies cross-domain for the forum - which is a 3rd party forum skinned for Yamaha. It's not for advertisement and this feature breaking other login credentials can be found in searches.
This issue is antithetical to the greatest web issue of our time: privacy.
Apple's doing a good job of attempting to make some level of privacy available to its users, and some control over the spying and nefarious activities of advertisers, which include the two most powerful companies of all-time: Google and Facebook.
In order to use this website, the worst aspect of their behaviour needs to be enabled on Apple devices.