Dual boot problem "Kernel too old"

Hi I had linuxcnc 2.7.14 wheezy for nearly half a year beside my Ubuntu 18.04.4 LTS installation. I use UEFI and grub. Sorry if use wrong terms as I am new to this. Now when I choose in boot menu to start Linuxcnc I get message "Kernel too old" and boot stops i also reinstalled Linuxcnc but also that didn't help. Ubuntu still works fine. So I guess something updated in Ubuntu that gives me that error message...
any ideas

thanks Felix

The RTAI kernel will only boot via grub, RTAI doesn't support booting via UEFI.
Basically UEFI and RTAI don't play well together, no idea why anything would of changed if the system was previously working.

You may have to elaborate a bit more on the history. Legacy\CSM is the option that will work with RTAI. As you mentioned Wheezy & Linuxcnc I'm assuing your Wheezy Installation is using a RTAI kernel, that assuming Whhezy has been installed via the official Linuxcnc image.
More details regarding the initial installation, was Wheezy or Ubuntu installed first ?
What medium was used to install wheezy ?
Details details details.

I try to give as much details as possible. But hard to say what's important. Yes I used the official LinuxCnc iso from the website. I used an USB sticks to create both systems. Bother from windows with USB live. First installed LinuxCnc then Ubuntu. Ran update-grub to make LinuxCnc visible in boot entry. Later removed LinuxCnc when I realized the error with kernel too old and reinstalled it. I hope that helps

When you booted and installed Wheezy was that booting with UEFI or legacy ?
When you booted and installed Ubuntu was that booting with UEFI or legacy ?

In bios legacy support is enabled
For LinuxCNC I don't know but I didn't change anything in bios. So I guess both where installed the same way. Which is UEFI for Ubuntu.
So you think when I reinstall both and disable UEFI it could work maybe. That would be easy

Yep that should work. If there is a option to have BIOS only or completely diasble UEFI pick that.
Install Wheezy, first install grub to the default destination, play around with it for a bit then post back here. There's some trickery with grub involved next when installing ubuntu, but I'm working on something atm so will need some time to post what you have to do next.
What I'm going to suggest is that the way grub is installed, when you update grub it will ignore the OS on the other partiton, but there will be an entry to sort things out.
The down side being an extra keypress to load the OS that is installed second.

Haven't found the entry yet in bios. Only secure boot and legacy support. But will dig into it. Then I'll gonna do as you suggested. Thanks stay safe
Felix

Run with legacy

My bios configuration was the whole time like that
Legacy support enabled
Secure boot disabled

But when I run in Ubuntu
[ -d /sys/firmware/efi ] && echo "EFI boot on HDD" || echo "Legacy boot on HDD"

I get EFI boot on HDD

So i managed to enter Wheezy LinuxCnc without reinstalltion. i will describe the path that lead me there. Knowing that this is not the way to properly start it. But maybe this gives someone the information on how to do it properly.
First i start the computer to the grub menu where i can choose between Ubuntu and Wheezy. On top it says Grub 2.02. When I press ESC I get to the Grub 2.02 shell. Without doing anything there I type exit. Now I don't get back to the grub 2.02 menu. But instead i get to a menu which says something Grub 1.99... In that menu only Wheezy is listed and not Ubuntu. When i start Wheezy it works fine.

So I guess I have two different Grub version one which is installed from Ubuntu and is called from UEFI. And one which is installed from Wheezy and runs without UEFI. Does that makes sense. And how could I get from there?