"Error 500" when trying to access website

Try to deactivate the extensions you installed, restart your browser and see if you can log in your online account. If so, then, try to isolate the problematic extension (by reactivating half of them and so on).

And if that doesn't work, delete you cookie(s) for that site and try again (allowing a new cookies).

(Sometimes the NoScript addon also can give you issues... Hate to say so but I've seen it, more than once. Despite that it's an excellent addon I run every day.)

Thanks very much for the suggestions. Unfortunately neither of them worked.

I'm running Trisquel 6.01. I will try a 7.0 live dvd and see if that makes a difference.

Nope, no difference.

Trisquel 6.01 live dvd: successful login. Hmmmm.......

Until the first Abrowser upgrade. Login unsuccessful.

500 is a vague server error message, maybe the problem is on the other side? Or can you consistently fail on one setup while making it on another at the same time?

https://en.wikipedia.org/wiki/HTTP_errors#5xx_Server_Error

Fails consistently on upgraded 6.01. Consistently successful on 6.01 without upgrades.
Upgraded packages (Abrowser version 28.0 to 41.0): libnspr4, libnss3, libnss3-3d.
When I try to force earlier versions of these packages, Abrowser is completely removed and has to be reinstalled.

When I force Abrowser version 19.0 over 41.0 (Packages installed during upgrade: abrowser-globalmenu, xul-ext-ubufox) login successful.

And in the bigger scheme of things, it matters not too much at all really. When I feel the "need" to access the website I can use a Trisquel 6.01 live dvd that is "not updated". Happy hacking!