ReillyBrogan
Thanks for your reply! Unfortunately, that didn't help. Still the old UUID in the boot parameters...

    So to be clear you ran sudo blkid, got the UUID of your swap partition, and then copied that into your /etc/kernel/cmdline.d/10_resume.conf file and ran sudo usysconf run -f? If so that should be all you need and the only issue I can foresee is if your system has another partition that's being detected as the boot partition for some reason.

      ReillyBrogan
      Yes, that is what I did. This is what it looks like looks like.

      $ sudo blkid
      Password:
      /dev/nvme0n1p3: UUID="8003e745-71c4-40ad-9d1b-32c84ab19399" BLOCK_SIZE="4096" TYPE="ext4" PARTUUID="9dcb432c-4ce6-4bd7-9979-5b0177eafbf9"
      /dev/nvme0n1p1: UUID="0960-25E5" BLOCK_SIZE="512" TYPE="vfat" PARTUUID="5400f121-eba1-4c8b-aaa9-9a3f40332bfa"
      /dev/nvme0n1p2: UUID="75b79613-ca67-4e79-bd33-1e32e8233658" TYPE="swap" PARTUUID="d41f3d54-7381-4c2b-8cda-0d364fc4b098"
      $ cat /etc/kernel/cmdline.d/10_resume.conf
      resume=UUID=75b79613-ca67-4e79-bd33-1e32e8233658
      $ cat /boot/loader/entries/Solus-current-5.15.77-219.conf
      title Solus 4.3 Fortitude
      linux /EFI/com.solus-project/kernel-com.solus-project.current.5.15.77-219
      initrd /EFI/com.solus-project/initrd-com.solus-project.current.5.15.77-219
      options root=PARTUUID=9dcb432c-4ce6-4bd7-9979-5b0177eafbf9 quiet loglevel=3 splash systemd.show_status=false rw radeon.si_support=0 radeon.cik_support=0 amdgpu.si_support=1 amdgpu.cik_support=1 ignore_loglevel systemd.show_status=true splash=silent resume=UUID=75b79613-ca67-4e79-bd33-1e32e8233658

      I also have not idea how to troubleshoot this. Do you have any hints for me?

        Unfortunately not on boot. I boot, press the space bar repeatedly and when the menu is appearing, I press 'e'. The shown boot parameters include resume=UUID=64d16.... (I would upload an image but that doesn't seem to work for me right now). Is there maybe an additional storage location of the boot parameters? A backup or something?

        Wait, is your boot partition even mounted? Run sudo mount and look for /dev/nvme0n1p1 on /boot

          ReillyBrogan I always oversee the stupid stuff -_- It was not mounted. I mounted it, ran sudo usysconf run -f again, and now everything is working fine!
          sudo mount did not mount /dev/nvme0n1p1, i had to mount it manually. Should it do it automatically? Is there supposed to be an entry in fstab?

            No, the boot partition is not supposed to be mounted normally. Usysconf is supposed to mount it temporarily during updates but then is supposed to unmount it afterwards.

            What likely happened here is that usysconf/clr-boot-manager will assume the partition is already mounted if there are already files in it and will perform all operations on the /boot folder as normal (which then are ignored since the actual boot partition is used). I think this can happen if you have the boot folder open in a terminal or somewhere and the mount fails, which is probably what happened in this case.

            I'd recommend that you unmount the boot partition and delete everything under /boot (but not the folder itself). This will ensure that usysconf keeps it updated in the future.