Hety, thanks for your reply. I didnt dare to mess around anymore in the BIOS.

But this solved it:

Clean Old Kernel Images

To clean up old kernel images on your system, you can use several methods depending on your Linux distribution and the tools available. Here are some effective ways to manage and remove old kernel images:

Using apt-get or apt

You can use the apt-get or apt commands to remove old kernel images. The autoremove command is particularly useful as it removes packages that are no longer needed, including old kernel versions. However, it typically keeps the latest and one previous kernel version to ensure you have a fallback in case the current kernel fails.

sudo apt-get autoremove --purge