On architectures that require a 2 stage bootloader, such as i386, the second stage boot loader can use the boot.conf(5) configuration file to boot a kernel from any FFS filesystem in the OpenBSD disklabel from any drive.
But the bootload can only find that /etc/boot.conf file on the FFS filesystem in partition "a" of the booting drive.
The kernel also sets the root partition, needed for finding /etc/rc and starting normal execution. If this is not "a" from the booting drive, this requires either a custom kernel or manual setting during boot via the "boot -a" second stage bootloader command.
If editing the disklabel to swap partition letters is not an option, re-install certainly is. It is not clear to me how the kernel ended up on "e", but it should not be there.