Some possibilities:
https://support.mozilla.org/en-US/questions/1203686
May vary on browser and antivirus compatibilities. Is the version of Avast current?
Found this regarding a similar error message:
https://support.mozilla.org/en-US/questions/1203686
My suggestion would be to clear the cookies and old cache. Make sure the browser being used is a current release. The OS is also up to date including security patches. If Avast still shows an error message there may be a security setting that doesn't like the forum but not necessarily does this imply the forum is an issue.