500 JHtml jemhtml not found

Hi thanks for trying to clear this up..
a) I'm currently set for 'Default (Responsive Style)' No to 'Use Icon Font' - this leads to the '500 JHtml jemhtml not found' when trying to edit replicating events.
b) Changing to 'Default (Responsive Style)' Yes to 'Use Icon Font' - does NOT solve the issue.
c) Changing to 'Legacy (Table Style)' No to 'Use Icon Font' causes any page that uses the Jem-Module-Banner module to throw up an error (see attachment 'error1')
d) Changing to 'Legacy (Table Style)' Yes to 'Use Icon Font' causes any page that uses the Jem-Module-Banner module to throw up an error (see attachment 'error1')

As an aside I use the {module [123]} code inside articles to shown specific events (these refer to 'Jem-Module-Basic' modules) these now show the small diary icon rather than the event image that it used to do. I don't know if this shines any light on the problem?

Thanks for your responses..

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