Your LimeSurvey version: Version 3.17.13+190824
Own server

==================Hi,
I can't edit sub-questions. When I click on "edit sub-questions", a blank page appears.
It was working fine before. Last month my colleague created a questionnaire and she had no problem.
Can someone help me to solve this problem?
I contacted support who can't help me because I'm using the "community edition".
Thanks in advance for your help !

You could start by updating to the latest 3.x version. Yours is 4 years old.