I am having the same issue. Verizon FIOS needs to contact Mint Bill to address the issue. Verizon FIOS new logon process is not compatible with Mint BIll access to our Verizon Fios account.
Verizon FIOS when can we expect a fix?
I've had this issue was recently able to resolve it.
There's 2 Verizon account types: Wireless & Residential.
The issue is the security question used when trying to log into the residential account. This is not present for the wireless type.
Delete the current account type, and re-add. Make sure you select the residential account. That security question portion my not be present at the initial screen where you just put in the username and password. It will, however, show up during the process.
Hope this helps.