The fact that I plugged in from the same output into a different device later that showed the same symptom whereas the kabinet was still plugged in and working felt like that output had a fault.
To me, this would really imply grounding issues between different AC powered equipment. Kabinet is passive device after all without own grounding and being able to take some 200W of 'abuse' even.
Reminds somewhat the 'classic' "Got electrical shock when I touched microphone" -issue where gear have differing ground potential which escalates as electric spike once shorted. In the microphone case the shorting piece is the person itself touching strings and mic grill simultaneously, but here the triggering factor would be the XLR cable itself.
So would look into the possibly incompatible (difference in ground potentials), ungrounded or faulty AC outlets.
Hopefully the pros find the root cause and get the issues sorted out for you. Be careful until then though.