In the orange box, enter your email address (not your forum ID) and then your password in the blue box.
You should now see a Forum Dashboard screen and a link to the Beatle Forum via your username; e.g TheEnd
Click the username link - that'll sign you into the Beatle Forum and open up your Profile page.
Now, depending on your browser, from then on you should be able to use the log-in link on the Beatle Forum homepage but, if that fails, use the other link: login.proboards.com/ as that will definitely get you in.
I tried logging into the forum using various browsers on my Mac and found that Firefox didn't work but Chrome and Safari did. And I've just checked on the Proboards forum on this issue and there is a known bug which they are trying to sort out. The good news is you definitely can still get into the Beatle Forum using that other link