I can't say for certain, but we've been having funky problems with Comcast / Xfinity. We've had various customers get unstuck by..

1) Turning off protected browsing

2) Turning off 'Xfinity HotSpot'

3) Someone did this: Xfinity has an instruction on how to disable it.  1) Need to sign in to your xfinity account. 2) Select xfinity xFi from your services . 3) On next screen select 'More' on the upper right hand corner of the screen. 4) Next screen shows More Options, select 'My services'.  5) the next screen will give you the option to turn off /on Advanced Security.

4) We changed something on our end with our certificate

I don't know if this is the same issue as Lacerte, but we have the following check:

Try this url in a browser: 

https://ftpfp.proseries.com/pub/proseries/check.txt

it should say "connection to akamai... ok". If it doesn't.. then for sure you are dealing with this issue. If it does say that, it still might be an issue with lacerte.com or whatever it's using to install / get updates.

The easiest way to tell if it's Comcast.. use a different internet connection entirely to verify.  For example turn off wifi access on your phone, tether it and use it's connection; use a neighbors wifi, or carry the computer/laptop over to a non comcast house and try from there.  I know those are all pains..but probably less painful than calling Comcast.