Any suggestions?

The only way to get this mounted is manually:
With 'grub-mount' from command line.

# Some background remarks:

grub itself seems not able to boot it
with 'kobenbsd …'.

And the EFI-Bootloader of OpenBSD doesn't find its own
partitions.

But OpenBSD's installer installs its own OS perfectly.

I'm running the most up-to-date version of
KDE neon User Edition: 5.9.5.

OpenBSD is: 6.1.

Is this GRUB's fault? And therefore KDE neon's ?

Or is this broken EFI-Hardware?

Please, don't tell me to go to OpenBSD-Forum first.
Nothing helpful to be found there.

# End of remarks.

Those are the questions coming up for the user:

Is KDE neon unable to mount OpenBSD partitions in
the Plasma-GUI? Plasma fault? Kernel fault?

###

dmesg gives:

[ 627.289380] ufs: ufs was compiled with read-only support, can't be mounted as read-write
[ 627.289547] ufs: You didn't specify the type of your ufs filesystem

mount -t ufs -o ufstype=sun|sunx86|44bsd|ufs2|5xbsd|old|hp|nextstep|nextstep-cd|openstep ...

>>>WARNING<<< Wrong ufstype may corrupt your filesystem, default is ufstype=old
[ 627.289551] ufs: failed to set blocksize

#

My main point:

Shouldn't this be reflected somehow within the Device Notifier's user interface?
"Could not mount" seems a little short.

Shouldn't it be possible to enter the required mount options somewhere in
"System Setting" GUI?

Shoudn't it be mounted at least read-only by trying to use 'grub-mount' as fallback?

Because: The kernel messages tell us it would have been mounted read-only, anyway.
And although the kernel can't mount: 'grub' still can, nonetheless.