Today I received two reports of people getting blocked from particular pages on our password protected site—same reason: HTML:ChaseBank-A [Phish]
I reported on the Avast false positives report page.
How can I check our site for problems?
Do I need to tell each end user that gets blocked to manually update their Avast software?
First, you are posting after an avast team member has acknowledged this is an FP (False Positive) so you shouldn't need to check your site for problems.
Second, the Virus Definitions Updates happen automatically during the course of the day (several of them). In instances like this where people are actually experiencing this and reporting it in the forums, the advice would be to manually update the virus definitions. For those not experiencing it first hand (haven't visited your site recently) will get the update automatically.