When I click on a mob to attack it, it disappears. Tried relogging, tried rebooting.

Did rebooting help? I had the same issue yesterday with my level 3 mystic kobold with a rattling skeletton near Fort Atla. The mob disappeared after I pulled it with a DD. When I pulled the next mob, it appeard again and I had to handle 2 mobs. Then I quit DAOC (without rebooting my Windows 10 machine) and later started it (via the "Play" button on the Uthgard website) again and logged another char. The issue did not happen again.