Error: Incorrect DATE value: '23:45: 30'

 If you want help in this forum, you must give more informations about your system: Joomla version, JEM version. What did you change?
I can't reproduce your issue in my sites, and had never this time format! I am now on J5.3.4 and JEM4.3.4
jojo12
EDIT: 
perhaps it helps, if you reinstall Joomla and/or JEM over your existing version.

Hi JoJo12,

I run the homepage with Joomla 5 and JE: M in the current version. Reinstalling the JE: M component did not avoid the error message.
In addition, further JE: M modules, such as "JEM-Wide Module" and "JEM-Basic Module" cannot be displayed on the "sidebar-right-1" position?
However, a "Own module" is displayed?

Step-by-step means:
you want to create a new event, in backend or frontend or both, which drop-down do you use (start date, end date, start publishing, end publishing...)
If noone can reproduce your issue, then it must be a problem of you! 
 

Do you set a special recurrence?  Why do you write: COM_JEM_WRONG_EVENTRECURRENCE_WEEKDAY? Is it showing this?

@aray Could you send me a screenshot of the recurrence settings for this event?
I think this information could help you.