Hi Imad!
Good to hear again from you! Thanks for coming back and letting us know about that.
First, I will kindly ask you to update to our latest versions of our plugins and the WordPress core.
Then, could you please enable WP_DEBUG and WP_DEBUG_LOG ? This will create a debug.log file inside the /wp-content directory.
After that, try navigating a few times into the page where you are having the service unavailable message, so that in case there is an error, it will be saved into the debug.log file. Please share that log file with us so we can spot the problem from there.
Also, are you on a shared hosting or VPS?
Just to set expectations right, we cannot accept login credentials as stated in our terms here > https://theeventscalendar.com/terms/
Thanks,
Victor