Sometimes MySQL needs more open files than cPanel's automatic algorithm predicts. In those cases, you can set the limit higher manually.
Turn off cPanel's automatic setting of the value in Tweak Settings by following the steps in this article: Changes to MySQL configuration are reverted automatically
Edit the file at /etc/systemd/system/mysqld.service.d/limits.conf with the following command:
systemctl edit mysqld
Tell systemd to re-read the config files to apply the changes with the following command:
Restart MySQL with the following command: