You could temporarily solve it with an internal rewrite.
Turn SEF on. Hover your mouse over the 'About' link. Do NOT click it. Hover your mouse over the 'About' link and look at the URL displayed in the browser status bar. The browser status bar is the one at the BOTTOM of the window. Look at the URL shown in the browser status bar.
Make a note of what it says after the domain name. Discard the first slash.
Turn SEF off. Click on the 'About' link. Look at the browser 'Address' bar. The browser 'Address' bar is the one at the top of the window.
Make a note of what it says after the domain name. Include the first slash.
Add this code to your .htaccess file, immediately BEFORE the Joomla SEF rewrite code:
RewriteRule ^stuff-in-the-browser-status-bar$ /index.php?stuff-in-the-browser-address-bar [L]
Now when you turn SEF on and click the 'About' link, the rewrite will silently connect your URL request with the internal script that will deliver the content.