Just from bits of documentation on 68k series processors:
An address error exception occurs when the processor
attempts to read a 16-bit word or a 32-bit Iongword at an
odd address. Attempting to read a word at an odd address
would require two accesses to memory - - one to access
the odd byte of an operand and the other to access the
even byte at the next address.
from http://marc.retronik.fr/motorola/68K/68000/Exception_handling_in_the_68000_[A.CLEMENT_1986_19p].pdf
I was pretty sure it's a 68k since most of HP's instruments from that era had one, and looking at the 54501a firmware it has strings for the other standard 68k exceptions (see attachment).
It should be possible to glean more information from the numbers on screen (they show the state of various CPU registers at the time of the exception), but it'd require disassembling & reverse-engineering the firmware image.