A bit of background-info maybe;
AO ships with 2 CSS-minification components. Depending on configuration it loads the default one (YUI CSS compressor PHP port) or the other (the legacy one, from an old versoin of Mr Clay’s minify, exceptionally, if the AUTOPTIMIZE_LEGACY_MINIFIERS constant is set).
Now while trying to decide which CSS minification component to load, AO also checks if there is already a CSS minification-component active (from the theme or another plugin). If there is, AO does not try to load it’s own CSSmin-instance (which would cause an error as well), but uses the existing one. This is the case here, with the external-to-AO CSSmin not expecting/ accepting the 2nd parameter.
The error was due to a mostly “cosmetic” change proposed by a contributor on GitHub and seemed OK when testing. Except it couldn’t pop up in tests, as this only occurs if the specific (version of the) external CSSmin component provided by the theme or another plugin doesn’t expect that 2nd parameter.
That’ll teach me not to try to fix things that aren’t broken ??
frank