When using the "Smart Layers by AddThis" plugin (version 1.0.8), in conjunction with wp-super-cache, it prevents the caching plugin from creating .html files in their respective directory.
This means that we only get the "wp-cache" feature and not the "wp-super-cache" feature of the plugin.
From looking at the debug of wp-super-cache, I suspect the error is connected to this massage:
"Supercache disabled: GET or feed detected or disabled by config." to be shown."
And this seems to relate to this bug report on a similar plugin:
Yours service seems great, but without having it work with wp-super-cache, it would be a real problem for me to use.
(p.s: my server is currently using PHP 2, I did not check this problem for PHP3)