See how to replace/reseal the oil filler pipe here:
Oil Seperator and Oil Filler Pipe Replacement
FWIW, I'm nearly certain you don't have an AOS problem. It's totally normal for a Boxster to puff a little smoke on initial startup. Since your's does it very infrequently, it's normal and nothing to worry about. If you had a bad AOS, your car would puke a large volume of smoke, and continue doing it for > 10 seconds at startup.
As for the oil filler pipe, you may find that it has been sealed with a piece of worthless fabric tape at the joint where you identified the clamp. Simply seperating the two segments at that clamp and applying some grey RTV may be all you need to correct the leak. The pipes probably haven't cracked.
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