I work in IT. My favorite thing to call tricky problems is "interesting." It always gives the reasonably frustrated client a reason to chuckle. "Very cool," or "Is -that- so" or "ah, good, a different error! Progress..." Are other ones to sprinkle in where they fit. Being upbeat while sharing their frustration to an empathetic degree buys you a lot more time to troubleshoot than if you sat on mute for 30min googling things and clicking on their remote window once every 5min to get the same error. And conveying that a problem is very complex is also important, when to them, going from not printing to printing seems like it should be an easy fix because "it worked this morning."