Illegal string offset 'ConvertPlug'

Hello,
I've just installed ConvertPlug and I'm getting this error. Showing on all pages.
Any ideas on how to get rid of it?
Warning: Illegal string offset 'ConvertPlug' in /var/sites/s/snowwatch.org.uk/public_html/wp-content/plugins/convertplug/convertplug.php on line 1331