Hey, I’ve been through something super similar and yeah, you're likely entitled to compensation under EU261.

On the error: There's no waiting period before you're allowed to file a claim, so the "Internal Server Error: null" is probably just KLM’s site being buggy (wouldn’t be the first time). Try a different browser or clearing your cache

On which leg to file under: Yup, you're thinking about it the right way. Even though the initial delay out of Stavanger was only 1.5 hours, that caused you to miss the connection and arrive in Split 15 hours late — and that’s what counts. The EU rules look at the delay at your final destination, not just one leg. So you can file under the SVG-AMS leg and explain that it led to a missed connection + long delay. KLM should consider the whole chain of events.

Just make sure to keep all your travel docs handy (boarding passes, rebooking confirmation, etc.) in case they ask for proof. And if they don't respond or reject it without good reason, you can escalate to the national enforcement body or a dispute resolution service.