Hey @Nicolas,
Thanks for reaching out!
I’m unable to reproduce this problem, as shown in this little screen recording from my test site → https://cloudup.com/cr0ppZSMS2s
So you may have a theme or plugin conflict here.
First, update your versions of our plugins to their most recent versions. This is 4.3.3 for The Events Calendar and Events Calendar Pro, for example.
Once everything’s up to date, if issues persist try doing the complete set of steps listed on this page → https://theeventscalendar.com/knowledgebase/testing-for-conflicts/
☝️ After EACH STEP in that process, go back to an edit-event screen and see if the tabs issues persists. Does anything change along the way when you do these tests?
Let us know what you find! 😀
— George