In case of convirting to an Opptunity from an appointment, error occurs following as repro steps,

1. create an appointment and save
2. Convert To>To Opportunity
3. check on "Open the new opportunity"
4. uncheck "Change the Appointment status to Completed and close the form"

Actual:
popup below message
"Your changes have not been saved. To stay on the page so that you can save your changes, click Cancel."
and click cancel, then error occurs only below comment.

"An error has occured."

unable to download log

Expect:
please fix it not to error or message clearly