HP Pavilion doesn't boot after Linux installation

[PeterBMS]

Hello,
I own an 8-year-old HP Pavilion desktop (SN 4CE6411JCL). Until now, I've been running Windows 10. According to Microsoft, upgrading to Windows 11 isn't possible because the CPU is no longer supported, even though the PC meets all the technical requirements for Windows 11.
I therefore decided to switch to Linux and created a bootable USB stick with Linux Mint Cinnamon Zara. I was able to use this to boot Linux on the PC and test it out a bit. Afterward, I started the permanent installation on the PC (with Linux as the sole operating system—no more Windows). I have two hard drives mounted on the PC: 1 128 GB SDD—where I installed Linux (formatted with ext4). 1 1 TB HDD (data) I left unchanged, using NTFS. However, a problem occurred when I booted Linux for the first time:
"Could not create MokListRT: Volume Full
Could not create MokListXRT: Volume Full
Could not create SbatLevelRT: Volume Full
Could not create MokListTrustedRT: Volume Full
Something has gone seriously wrong: import_mok_state() Volume Full".
I suspected a partitioning error and booted Linux from the USB stick again. But the same error message appeared.
Then I searched various forums and found a reference to the so-called "secure boot," which should be disabled for Linux. I tried that several times. To do this, I disabled "Secure Boot" and enabled "Legacy Support" in the BIOS setup (F10) under Security BIOS, and another time I also disabled Legacy.
In both cases, a "Changing the Operating System Boot Mode" window appears after booting. It's noticeable that above this window initially a window for adjusting the OS Resolution appears. However, it doesn't respond to any input. That might be a different problem.
The window disappears after a while, and I can enter the password to accept the change and confirm it with Enter. But after rebooting, the "Volume Full" error messages appears again, and when I check the BIOS, "Secure Boot" is enabled again and Legacy is disabled. I also tried it once without a password (ESC = Continue without change). But, as expected, that didn't work either. So, I can't currently boot from either the SSD or the USB flash drive. Changes to "Secure Boot" aren't taking effect.
What am I doing wrong?
What can I do?

Peter

Specification:
Desktop: HP Pavilion Desktop 560-p0xx
SN: 4CE6411JCL
First switch on: 12/18/2016
System platine ID: 828A
BIOS Revision: 828A vF.14
CPU: Intel Core i5-6400 CPU @ 2.700 GHz
RAM: 8192 MB DDR4 / 2133 MHz / Single Channel
L1/L2/L3 Cache: 64kB x 4 / 256 kB x 4 / 6144 kB x 4
channel A: XMM1 0 MB
channel B: XMMO 8192 MB
HDDs:
SATA0 128.0 GB, SanDisk SD7SB6S-128G-1006
SATA1 1.0 TB, ST1000DM003-1SB102
USB 62.9 GB, Generic Mass Storage
CD-ROM: SATA2 hp HLDS DVDRW GUD1N
SATA Emulation: AHCI