Trying to view a forum without the capability may lead you to a broken page.

Reproduce:

1) View a forum ( mod/forum/view.php?id=xxx ) with a user that does not have the correct capability ( mod/forum:viewdiscussion )

2) See the message 'You do not have the permission to view discussions in this forum'.

3) Click the 'Continue' button.

4) Error: Invalid parameter

Cause:

The button on step 3 does not have a valid link, so it redirects to the same page without the query string '?id=xxx'. The error comes from the missing query string.

Suggested solution:

Redirect to $CFG->wwwroot.'/index.php' as usual.