I can't think of anything reasonable for this, especially since it works on another computer.
You may want to check video card drivers, either update to the one the coworker has if it's the same system specs or update to the latest or to the ptc certified driver for that card if it's a certified card. Video card issue cause some really unexpected behaviors sometimes so it's my only guess.
Other thing is, you say you went to default config.pro. You may want to make sure by checking what config.pro files are loading. You can do that under FILE -HELP - SYSTEM INFORMATION. There is a section called configuration information and it lists all the config files read. IF POSSIBLE, temporarily rename all of those to clean up any possible settings that could potentially cause problems.