I want to upgrade from Leap 15.1 to 15.2 on a machine with the following device layout
> lsblk
NAME                                       MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
sda                                          8:0    0 149.1G  0 disk
├─sda1                                       8:1    0   156M  0 part  /boot
├─sda2                                       8:2    0    22G  0 part
│ └─cr_ata-WDC_WD1600BEVT-22ZCT0_WD-WXE708N10973-part2
│                                          254:0    0    22G  0 crypt
│   ├─system-root                          254:1    0  19.9G  0 lvm   /
│   └─system-swap                          254:2    0     2G  0 lvm   [SWAP]
└─sda4                                       8:4    0   127G  0 part
                                           254:3    0   127G  0 crypt
    └─system-home                          254:4    0   127G  0 lvm   /home
sr0                                         11:0    1  1024M  0 rom
Disk space usage is reported as
> df -h
Filesystem               Size  Used Avail Use% Mounted on
devtmpfs                 2.0G     0  2.0G   0% /dev
tmpfs                    2.0G     0  2.0G   0% /dev/shm
tmpfs                    2.0G  1.5M  2.0G   1% /run
tmpfs                    2.0G     0  2.0G   0% /sys/fs/cgroup
/dev/mapper/system-root   20G   13G  5.8G  69% /
/dev/sda1                148M   42M   96M  31% /boot
/dev/mapper/system-home  125G   49G   71G  41% /home
tmpfs                    393M   12K  393M   1% /run/user/1000
This layout has been used ever since openSuSE 13.2, where it was suggested by the installer. The /boot partition is not too large, but I always managed to update kernel versions, with keeping the running and one older kernel.
Now I use the USB installer downloaded from openSuSE, and it works fine until I receive the system probing screen, when it reports that I am out of disc space ob /boot (160% usage). Booting back into Leap 15.1, I deleted the older kernel version to gain some space, and repeated the attempt to upgrade. The same error turns up, now the usage of /boot is reported to be reduced to 123%, but still more then 100%. I can't provide a screen shot during the upgrade, but here is the contents of the pop-up window shown:
Disk Space Warning - Yast2
Error: Out of disk space!
                           Free         Total
/boot    123%       -27.2 MiB   120.1 MiB
/          71%         5.0 GiB      17.6 GiB
/home  41%         69.1 GiB    117.5 GiB
However, device /dev/sda1 mounted on /boot still is at only 31% according to df -h. The installer probably adds up the memory already used and the additional one that will be taken up by the new version, for /boot the kernel still left from Leap 15.1 + the new one from the Leap 15.2 upgrade, but that should not add up to 123% if the new kernel is not much larger in size than the previous ones. Something seems to be inconsistent here.

Is there something wrong in the disk space calculation in the installer? Anyway, what can be done about this, except doing away with the upgrade and starting a totally new install of the machine with 15.2.

Thanks for any help.