Testing iCagenda 4.02 (free version) on top of Joomla 5.4.1. and PHP 8.4.17.
The environment which has been upgraded from Jooml 4 works fine as long as I keep confirmation Emails to both, Admin and registering user, switched OFF.
As soon as I enable one of these mail functions in the backend the registration process gets stuck after submission and  ends with this frontend message:
EDIT: image was not properly uploaded, so not visible to public, only moderator

The long cryptic identifier in the last line differs during my tests.
However, a correct booking entry is created and after some delay the (customized) confirmation Email is sent to the registrant, not to the admin though.
On Joomla 4 emails worked fine. The Joomla 5 Logs do not show entries that may help.

Has anyone hints how to further track / fix this issue? 
 

Joomla version is  5.4.2. 
But it shouldn't matter, since I've meanwhile figured out the issue: It has not been caused by iCagenda, but just became obvious by it.
In fact our upgrade Joomla 4 to 5 had for some reason changed the Email settings of our server environment. Was no more SMTP and Ports were wrong.
Once corrected in the Joomla backend (System -> configuration -> server tab),  the confirmation emails again go out as intended.

Thx and rgds, Matthias