Indeed, the Arabesque 600 is pre-SIS. Fairly certain in my reading that a number of changes occured when Shimano introduced the SIS line from their older derailleurs. Including a floating jockey wheel and some changes to the cable pull.

Zorro's mention of the ultra-6 is possible, but I couldn't find any mention of a HG20 freewheel in the ultra configuration. If your freewheel is indeed a newer Shimano HG20 model then this likely isn't the problem.

You've got two choices: run the shifters in friction mode with the current derailleur. Or get a SIS compatible derailleur that works with the light action shifters.