my PLL does not set "pll_locked" to 1 when I lower "pll_powerdown" to 0. The flag "PLL Auto Reset" is set in the IP core

What are you connecting pll_powerdown to? Are you using it with a Transceiver and is it connected to that? Is it connected to the correct reset controller?

The user guide states it "needs to be connected to a dynamically controlled signal". Don't expect it to work if you're driving it directly.

Cheers,

Alex