Thank you for your answers.
Trying to reproduce on Joomla! 3.9.20 Stable with php 7.4.7 working on t4_stark template. with default style and icon font =yes
Or in backend or in frontend I can create and edit recurrent events, without getting any errors you describe.
So my diagnose is: for some reason (I don't know why) seems corrupted! Did you change things inside of JEM, which could result in such errors?
Will try with the other settings too when I have more time