Burnt bottom.

The top of the loaf is exactly right. The base is black, hard, and bitter enough to cut off. Everything you did was correct except one thing, and it is almost certainly not the oven temperature.

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sourdough fixes

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1. most common

   Direct contact with a very hot pot base

   Cast iron holds an enormous amount of heat and dumps it straight into whatever touches it. Parchment alone is barely any insulation. The base of the loaf is effectively being griddled while the rest is being baked, and it browns several times faster.
2. very common

   Rack too low in the oven

   Most domestic ovens have their main heating element in the floor. A pot on the lowest rack sits directly in that radiant heat and gets hotter than the oven air temperature suggests. Moving up one or two rack positions changes the base temperature substantially.
3. common

   Preheat longer than the pot needs

   A 45-minute preheat is right for getting the pot to temperature. Leaving it in for 90 minutes does not make it hotter than the oven — but it does mean every part of the pot is thoroughly heat-saturated, including the base, with no cool spots left to absorb energy from the loaf.
4. sometimes

   Sugar or inclusions in the dough

   Enriched doughs, anything with honey or milk, and inclusions like dried fruit brown and then burn far faster than plain dough. If this only happens on your enriched loaves, the dough is the variable, not the oven.
5. occasionally

   An oven that runs hot

   Worth ruling out but rarely the whole story, because a genuinely over-hot oven burns the top too. If only the base is affected, look at contact and rack position first. An oven thermometer costs very little and settles the question.

1. Move the rack up one or two positions

   The cheapest fix and often the whole answer. Get the pot away from the bottom element and into the middle of the oven. Try this before buying anything.
2. Put a baking sheet on the rack below

   An empty sheet pan one level under the Dutch oven shields it from direct radiant heat off the floor element. Costs nothing if you already own one, and it works.
3. Double the parchment, or add a barrier

   Two layers of parchment help a little. A better version is a thin layer of semolina, cornmeal or rice flour between the parchment and the dough — it acts as tiny ball bearings of insulation and does not scorch the way the loaf does.
4. Drop the temperature after the lid comes off

   If you are baking at 500°F covered, go to 425–450°F uncovered rather than holding high heat for the whole bake. The loaf has done most of its expanding by then and only needs to finish and colour.
5. Lift the loaf off the base entirely

   For a stubborn oven: a small trivet, a folded strip of foil, or an inverted metal cookie cutter under the parchment creates an air gap. Slightly fiddly and it works completely.