Error in Semiologic 3 column theme

I am not quite sure what caused this error, but I get the following error code above my Semiologic template, when I open my Blog in both IE and Firefox browsers:

Warning: ob_start(): output handler ‘ob_gzhandler’ cannot be used after ‘URL-Rewriter’ in /home/market35/public_html/blog/wp-includes/functions.php on line 829

FYI my blog is located at https://www.marketingdefined.com/blog/

I noticed it in particular after installing a Contact Form plug-in “PXS Mail Form” available from https://www.phrixus.co.uk/pxsmail/ – however de-activating that plug-in does not help the error to go away.

Also if Dennis happens to see this – when trying to exclude the sidebar from showing on the main Pages of the website, does the following instruction still apply for version3.2 and the 3 column theme?

in footer.php, add a test before inserting sidebar.php, i.e

if (!is_page())
{
# require sidebar
}

Many thanks
Roseanne