After a freshly install of Hestia CP I got the error message Oops... Something went wrong! when accessing webmail.
After a couple of hours Googling I’d found that the error occurred because Roundcube needs permission to execute the PHP file inside the roundcube folder.
To do this, run the following commands:
chown -R hestiamail:hestiamail /etc/roundcube/
find /etc/roundcube/ -type f -iname "*php" -exec chmod 640 {} \;