I've discovered what can cause ALL browsers (other than PM) to be unable to login to the Avast forum.
Avast requires a new registration each year for users of FREE Avast. If a user happens to register using an email address different than the email address that matches the email address for the forum account and the user then understandably would create an Avast Account (my.avast.com) under the different (new) email address that they registered the Avast AV account for another year under then once the user is logged into that new Avast Account login (my.avast.com) using the different email address then the user cannot login to the forum. The user will find themselves not logged into the forum and the "login" link in the forum will not work as long as the user remains logged into Avast (my.avast.com) with the email that does not match the email address on file for the forum. What happens in this case when the user clicks on the login link in the forum is nothing.
For the forum to work the user must login using the email account that matches the email account that was originally registered for the forum account, but then the Avast account information will not be in sync with the Avast AV because the new registration is under the different (new) email account in this example. The way I look at it who cares if the Avast account information isn't the same as what it is with the other email address, this information has very little importance anyway because most of this information just amounts to Avast promotional stuff, the free account is still good for AV protection for a year which is what matters so the user should just login to their Avast account (my.avast.com) using the original email address that is on file for the forum to have the forum work as well and then everything that needs to work that really matters then does.
I don't think Avast thought this "all-in-one" login idea out very well. (every other forum is based on a UID and not an Email address and in the case of Avast this email address linkage complicates things in regards to the Avast account login requirements)
As to PM however, no matter what is done none of the Avast logins work including the Avast account (my.avast.com) login as well as the login link in the forum. What happens in any case with PM is that some kind of odd error type page occurs that consists of a partial page and an orange triangle when attempting to login in either case.
Of course using PM without the ability to login to the Avast forum posting is unavailable and therefore we can only view (to some extent) forum posts in a limited way (attachments fail to display unless logged in).
I thought I'd post this to help Avast users with Avast login problems as well as just in case this might offer insight into any ideas as to what can be done to help the Avast login work with PM.