I don’t have BBPress installed, but on my Tags archives pages I’m seeing this error:
Call to undefined function bbp_is_search() in …restrict-widgets\restrict-widgets.php on line 1621
Nothing else appears on that page.
I don’t have any of my widgets set to do anything special on Tags archives per Restrict Widgets, this just happens whenever the plugin is activated.
I think it’s because you’re using is_tax() to check if a user’s on a tag/taxonomy page. That conditional only works for custom taxonomies: http://codex.wordpress.org/Function_Reference/is_tax.
I was able to fix this by changing line 1607 to:
elseif(is_tax() || is_tag())
Thanks!