I've just discovered this moments ago by myself. Please consider looking into sharing this advice around for others. Maybe this problem is a unique one, nonetheless this can still be a problem for others if it just happened to me.
Thanks for your time and hopefully this can help somebody else in need.
Problem: Whenever I type in my login credentials, I have to wait for the Fido website to load and it takes forever. It continously says "Hang on... we're thinking." for a few minutes and it stays on the page loading and loading, basically nothing is happening and you're staring at the page. If this is done in other browsers, it continously says the same thing.
Note: I was only logging in using my mobile phone number but this issue could be done for e-mail addresses too, it's an access problem so I don't think it matters what username is used.
Probable Cause: Whenever you install an anti-virus and open up an internet browser, they might ask permission to install their own extension for that browser (to monitor websites and protect you). That antivirus extension might be the problem to why you're not logging in. It could be blocking you from accessing the website.
In my case: my anti-virus was Kaspersky and I've already got its extension installed on Google Chrome and Firefox which explains why I couldn't log-in on both browsers.
Will this work on other known anti-virus extensions for internet browsers? I'm not too sure. However this was a problem for me and possibly others who are in a similar situation like me.
Solution: Turn off your protection from that extension for the Fido website specifically or disable the extension itself if you can't find that option. Yes it's a risky thing to do so be sure to be on safe websites while attempting this.
If you turned off that extension for the website or disabled it, retry logging in and you should be able to get into My Account.