Have you tried on other phones
Have not tried that yet, only with my Laptop and this desktop computer I'm using at work.
or a mobile browser other than Safari?
This however, I did try. I downloaded the Firefox app (the browser I'm using now) on my iPhone and had the exact same result as with Safari, with the Forums telling me that my username/password are incorrect.
Have you tried telling the browser on your phone to request the desktop version of the site to see if that works?
This site is by default always in the desktop version on my phone.
How about clearing cookies/cache in Safari?
I did try that as well, no dice unfortunately.
I don't know what problem it could be with your actual user account.
The problem has to be with the forums, as it's the only one of my accounts that I'm unable to login to despite retaining the correct username & password. Amazon, Ebay, Yahoo, Facebook, Etsy, Fandango…every last one of my online accounts operates smoothly when accessed on mobile, and Arlong Park seems to be the lone bizarre exception.