I am running OpenSuse Leap 15 on a lenovo T440p.
After a call "zypper up" the system would not boot. It stop
with the prompt "grub>".
I speculate that the problem came from the fact that the directory
/home/pschmitt was 100% full.

(1) Following instructions found in the internet I did what
is required for booting with grub.
The system ended up in maintenance mode.
I learned that this is due to a wrong fstab. I removed the
mount instructions for fritz.nas. Since this did not help I also
removed the mount for a brtsf file system from fstab.
I later learned that this file system stores the snap files.
The effect was that the system did not boot at all.
(2) Now I was going for a new installation using another, older laptop
running Kubuntu I downloaded
openSUSE-Leap-15.2-DVD-x86_64.iso
and also
openSUSE-Leap-15.2-Rescue-CD-x86_64-Build31.307-Media.iso
and used the command

sudo dd bs=4M if=openSUSE-Leap-15.2-DVD-x86_64.iso of=/dev/sdb1
conv=fdatasync status=progress

to create a live usb disk.
Plugging this disk into the not-booting laptop and power it
up still ended as before with "grub>"
Also interupting the boot process and choosing the ubs disk
a temporary booting device did not change this.

I am desperate and at the end of my wits.

should be

This doesn't prevent Grub from booting. Your symptoms show that Grub is not even able to find its configuration file, which is not located on /home/pschmitt.
This is a huge step forward.
I think you should have copied the ISO to /dev/sdb, not /dev/sdb1. Before you do this, you also need to ensure that the USB drive is really identified by /dev/sdb. One method is to unplug it, run lsblk, plug it back in and run lsblk again. If /dev/sdb is not listed while it is unplugged, it's almost certain that this is the correct device file.
My guess is that you booted from the harddisk instead of the USB drive, because your computer did not recognize it as a bootable drive. Most likely because you used sdb1 as copy destination, or (less likely) you did not configure your computer to boot from USB.

I would try again with a newly created USB drive.

Thanks, both of you.
I created a new usb-disc using of=/dev/sdb
Using it on the non-booting laptop still brought up
the "grub>" prompt. This time I entered "exit" which
produced the usual first screen after powering up.
Among the possible choices of OS on the list there luckily
also was the Rescue system.
I followed the instructions how to use chroot and I am now
on the system. Internet connection is working.
I am now on a loss how to proceed. Zypper would update
some 1000 packages, but the updates fail. "reboot" is
not allowed as chroot.
I would appreciate any hint what to do, or what to read.

cheers Peter

There isn't enough information on you system to give any good advice, go here https://sourceforge.net/projects/boot-repair-cd/files/ download the iso put on usb, create a boot-info file and post.

I have run the boot-repair disc.
The report is available at
https://paste.ubuntu.com/p/5mwhsm2qFC/

I have not looked into it yet.
My fiddling around has revealed that the partition sda7 is almost full.
I have emptied the tmp directory on it.
This may be part of the problem, but more is required for a successful ending.

at the the grub prompt

I very much appreciate your help and I have the feeling that I am
getting somewhere. Here is what I did so far:

after entering
grub> configfile (hd0,6)/boot/grub2/grub.cfg

a screen pops up with four choices for OS.
The first

OpenSuse Leap 42.3

was selected. The last two carried "Windows" in their names.
The second one was

OpenSused Leap 42.3 advanced options.
I was too slow to select it and I am not sure if that
is the way to go.

Booting OpenSuse Leap 42.3 brought me again in maintenance mode.
But, admittedly much easier than I had done before.
After

cd ..
mount /dev/sda7 /home2

df
shows that this partition is 100% full. I mounted subdirectories
e.g.
mound --rbind /tmp /home2/tmp
to see what big files I could remove. But did not find anything that
changed to 100% memory consumption of sda7.

Try the entries in the advance options, one of them may get you back into your system.

Why not download the OpenSUSE Leap 15.2 installation DVD media, make a disc, and do an "upgrade" install? It will read your package info and re-install the OS, and should correct the issues for booting in the process. In the event that you DO have an issue with a filesystem / partition that is full, it should be able to alert you to that during the install as well.

Dear ember1205
yes, I am thinking of this option also. But, I am postponing it for the moment.
My recollection of previous updates is, that there remain a lot of additional things to do after it.
E.g. re-installing programs not loaded from repositories.
Is that different with "upgrade install" ?

Addition to my last reply.
I know that partition sda7 is 100% full. But, despite 3 different ways to
access the system in restricted mode I found no way to resolve this issue,.

If you use the same version of the openSUSE distro that's already on the box and do an "upgrade", it should not do anything with the installed software except to replace certain packages that may be missing or possibly corrupt. Anything else should simply remain as is. If you're running something other than 15.2, then use that version.

Answer to colorpurple21859

I have tried the entries in the advanced option selection

openSuse Leap 42.3 with Linux 4.12.14-lp150.12.7-default
openSuse Leap 42.3 with Linux 4.12.14-lp150.12.7-default with recovery mode
openSuse Leap 42.3 with Linux 4.12.14-lp150.11-default
openSuse Leap 42.3 with Linux 4.12.14-lp150.11-default with recovery mode

They all end up in maintenance mode
Option number 5: Windows 7 worked without a hitch. (I had not used in since 2017).

Checking dmesg after the OpenSuse boots I found

EXT4-fs (sda7) : VFS : Can't find ext4 file system

That sounds strange since sda7 contains a btrfs file system.
Running
btrfsck /dev/sda7
finds no problems with it and also finds a lot of free memory on it. Why does df say it is full?

I am considering the suggestion of ember1205 to "install upgrade" the Linux version that is currently in my
systems. What do you think of it?

You have part 7 as ext4 in fstab vs btrfs