The script defines the variable opera (and firefox and ie) to test later on which browser is executing it to use specific code. As expected, the variable was set to false. However, opera seems to provide some information about itself in an object named ‘opera'. Google compares the opera variable with “undefined” – but false is of course not “undefined”. I guess it is better not to use that name as variable, as it would probably break any script relying on the opera object when running under Opera.
The fix was to rename that variable to jsdoopera in all occurences. Now Google Analytics works.