Avast blocking applications

Avast Free Antivirus / Premium Security
1

Since downloading the latest version (3-5-2017), I cannot open cleanmgr (Error 0xc0000005), Task Manager, and even Sketchup. At boot-up I get a black screen, which can only be resolved with a restart (Task Manager can’t be accessed). I tried uninstalling and re-installing (using Avast Clear in safe mode), but the same problems persist. When I disable the Avast shields, all the programs load perfectly.

Windows 8.1.

How can I solve this?

2

Disable the behavior shield and see if it solves the problem.

3

Is this dangerous?

Also, now Avast is blocking Firefox as not a secure site!

4

That can’t be as FireFox is a browser, not a site.
Please attach a screenshot of the message to your next post.

5

You know what I mean.

Or in case you don’t: Error code: SEC_ERROR_UNKNOWN_ISSUER

Behavior shield is unchecked. Checked are: File System, Mail, and Web.