Can you please make sure that MySQL/MariaDB is running dine at the moment? If 3 of your sites are loading this error without you making any changes the issue should be related with the DB service itself.
Have you checked the MySQL error_log in order to see if the server is running out of memory or if the issue is with to many MySQL connections? As per the tutorial you can run this command in order to check if the server have recently run out of memory:
zgrep -a "allocate memory" /var/log/mysql/error.log*
zgrep will search through log files, including older log files that have been archived as compressed .tar.gz files. We’re searching for lines that contain allocate memory, in any error.log* file in the
You can always login and check the MySQL
error_log when the issue occurs using tail to either check the last 100 entries in the log or monitor the log in real time:
tail -n 100 /var/log/mysql/error.log
and monitor it in real time:
tail -f /var/log/mysql/error.log
Hope that this helps!