Can’t access wp-admin on site (https://thedavanos.000webhost.com/wp-admin) after upgrading to latest WP version, also can’t edit venue when adding events with Events Calendar plugin.
That would because the correct URL is infact http://thedavanos.000webhostapp.com/wp-admin 
Also I’ve enabled debugging to show errors on your website to allow you to troubleshoot with your developer easier.
It appears the issue is down to your theme
It appears your correct about the theme, this is the error message when I try to login:
Warning : Cannot modify header information - headers already sent by (output started at /storage/ssd2/110/10850110/public_html/wp-content/themes/music-lite/functions.php:361) in /storage/ssd2/110/10850110/public_html/wp-includes/pluggable.php on line 1251
Warning : Cannot modify header information - headers already sent by (output started at /storage/ssd2/110/10850110/public_html/wp-content/themes/music-lite/functions.php:361) in /storage/ssd2/110/10850110/public_html/wp-includes/pluggable.php on line 1254
I can access theme’s files via the File Manager but I’m not sure what I need to do, would appreciate any advice. Thanks for your help
I still can’t access the dashboard, any help would be much appreciated!
Sadly I cannot assist with theme or plugin issues only actual hosting issues.
I would recommend talking with your developer/script/theme author.