Monster By Mistake! (found CGI animated series; 1996-2003)
From The Lost Media Wiki
Monster By Mistake was a French-Canadian animated cartoon series that was broadcasted on YTV from 1999 to 2006. The plot of the series is siblings Warren and Tracey finding a spell book, which Tracey uses to accidentally transform Warren into a monster whenever he sneezes. When he sneezes again, he turns back into Warren. There is also a ghost who lives in the attic, who phases through doors to get into rooms, and who appears to be the spirit of a stereotypical 1950's gangster.
Information on the episodes are hard to come by except for a small synopsis on each episode on http://www.catapultproductions.com/Behind/index.html
Only 3 episodes have been uploaded to the internet, with the rest of the 52 episodes being lost.