Yesterday (Friday) afternoon, I encountered this same problem - when I attempted to login to my ProtonMail account, I encountered a threat message from my Avast antivirus software that aborted my connection to account.proton.me because it is infected with URL:Phishing.
I immediately contacted both AVAST and ProtonMail and advised each of them of this problem and requested their corrective action at the earliest possible time. Shortly afterwards, I was contacted by ProtonMail support and advised that they were already aware of this problem and were in contact with AVAST support - the issue appeared to be on Avast’s end and that both support teams were actively working to resolve this problem as soon as possible.
Fortunately, it appears that both support teams have been successful in their corrective action efforts. This afternoon (Saturday) at 2:01p ET, I was able to successfully log into my ProtonMail account!
I thought I would share the above information. Take care…