"invalid sector size 65535"

[dashmZ]

After setting up my PC with Linux using EFI and grub2 to boot, I installed 
OpenBSD on the same disk.

Booting OpenBSD shouldn't be that hard with grub2, in fact it took just a few 
lines (that's what I thought):

menuentry "OpenBSD" {
    set root=(hd0,gpt4)
    kopenbsd /bsd -r sd0a
}

As you can see below, the OpenBSD-partition is the 4th on the disk.

But as it turned out, this does not work. I'm always ending up with this error 
message from grub:

"invalid sector size 65535"

Since booting to my Linux works without any problems, I doubt that the problem 
lies in a incorrect grub2-installation. Also I'm pretty sure OpenBSD was 
installed correctly.

I couldn't find any solutions to this that would work for me. So I'm dropping 
some information about my disk and hope for hints (sorry for localized text 
from fdisk).

# gdisk /dev/sda
GPT fdisk (gdisk) version 1.0.1

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): p
Disk /dev/sda: 4883781168 sectors, 2.3 TiB
Logical sector size: 512 bytes
Disk identifier (GUID): E6D6031D-5D54-4BFE-866F-000F05E9DF78
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 4883781134
Partitions will be aligned on 2048-sector boundaries
Total free space is 3565 sectors (1.7 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048          206847   100.0 MiB   EF00  
   2          206848       105064447   50.0 GiB    8300  
   3       105064448      4799895551   2.2 TiB     8300  
   4      4799895552      4883779583   40.0 GiB    A600  puffy

# fdisk -l /dev/sda
Festplatte /dev/sda: 2,3 TiB, 2500495958016 Bytes, 4883781168 Sektoren
Einheiten: Sektoren von 1 * 512 = 512 Bytes
Sektorgröße (logisch/physikalisch): 512 Bytes / 4096 Bytes
E/A-Größe (minimal/optimal): 4096 Bytes / 4096 Bytes
Festplattenbezeichnungstyp: gpt
Festplattenbezeichner: E6D6031D-5D54-4BFE-866F-000F05E9DF78

Gerät          Anfang       Ende   Sektoren Größe Typ
/dev/sda1        2048     206847     204800  100M EFI-System
/dev/sda2      206848  105064447  104857600   50G Linux-Dateisystem
/dev/sda3   105064448 4799895551 4694831104  2,2T Linux-Dateisystem
/dev/sda4  4799895552 4883779583   83884032   40G OpenBSD-Daten