Dual Boot change to UEFI

I currently have a dual boot setup where I have Windows 10 on one disk and ubuntu on another. This was installed in CSM mode, not UEFI. It works great, when I start my PC I can select Ubuntu or Windows with the GRUB bootloader. Now I wanted to get ready for Windows 11, one requirement for Windows 11 is to enable "Secure Boot". In order to enable that I had to change to UEFI. The issue is, that with UEFI I cannot boot Ubuntu. In the boot options it only shows "Windowsbootmanager" and the only disk I can select is the one with windows on it. After switching back to CSM mode I can access windows and ubuntu.

Side notes from my research:

• My /boot/efi directory is empty.
• grub-install --version gave me: grub-install (GRUB) 2.04-1ubuntu26.13

After following instructions from the answers (creating a partition and using boot repair):

Now I see two options in my BIOS/UEFI, the Windows Boot Manager (Samsung SSD EVO 250GB) and ubuntu (Samsung SSD EVO 250GB) . If I select Ubuntu as first bootoption I receive the following error screen: Ubuntu GRUB Error Screen Now it is not possible anymore to boot ubuntu. Even if I switch back to CSM.

Here is the pastebin from boot repair: http://paste.ubuntu.com/p/4qhwYQt8k8/

I wonder if I have to convert the disk with ubuntu on it from MBR to GPT partition table. If so, how can I do it? I found the tool gdsik but as mentioned, my ubuntu doesn't boot anymore.

From the Ubuntu help:

Converting Ubuntu into UEFI or Legacy mode

Note: Do not follow this procedure if your computer is already booting correctly. Use this procedure only if you believe you've accidentally installed Ubuntu in BIOS/CSM/legacy mode and you want it to boot in UEFI/UEFI mode.

• Start Boot-Repair, click on "Advanced options", go to the "GRUB location" tab.

• If you do not see a "Separate/boot/efi partition`" line, this means that your PC does not have any UEFI partition. In this case, exit Boot-Repair, then create an UEFI partition (see the "Creating an UEFI partition" paragraph above).

• If you see a "Separate /boot/efi partition" line, tick it then click the "Apply" button.

• Set up your BIOS so that it boots the HDD in UEFI mode (see the Set up the BIOS in UEFI or Legacy mode paragraph above).

Skip 1 and 2 and go straight for "Creating an UEFI partition"

An ESP can be created via a recent version of GParted (the Gparted version included from the Live session is OK), and must have the following attributes:

• Mount point: /boot/efi (remark: no need to set this mount point when using the manual partitioning, the Ubuntu installer will detect it automatically)

Size: minimum 100Mib. 200MiB recommended.
Type: FAT32
Other: needs a "boot" flag.