I host multiple Joomla sites on a Single Google Computer Engine VM. It has been working very well, but after I performed a Joomla core update I suddenly found that those websites can't be accessed anymore. All websites show "Error" after inputting the domains in the URL.
Here are my websites:
Although I can't access the URL, I can connect the directory of websites with FTP and SSH client.
Other information: Joomla Version: 3.9.13; Hosting: GCP VM; Server: Ubuntu 18.04 with Apache2, MariaDB and PHP7.2
Can anyone help me to diagnose and/or fix this problem?