Have you had an email account up and running and then it stopped fetching emails and you are also getting a "API Error 500" when trying to send an email? And the SMTP is verified and you are not seeing any errors in the "Realtime"-section of the emails?
Then go to your Email -> Email Accounts section in the Connectel admin view and edit the Email account you are troublshooting. Have a look at the "List"-setting of that email as shown in the image below.
If you see that the setting is empty you need to select a list and then save the account using the bottom in the top right. This happened because the list previously selected for this email account was deleted.