When I book a meeting Outlook opens on my computer. I have disabled Outlook i the settings and advanced Options. Outlook is not my default Maill app either.
I can;t say to much without th details of your setup, but here is some steps you can try.
Are you using MS Office in your computer? If yes have you tried to rapair it? (control panel > Programms > Outlook > Modify )
Have you see which in the Windows default Programs if Windows using outlook as default for meetings?
Do you setup your meetings from Windows calendar?