MySQL server versions (5.7.41 and 8.0.32) introduced a bug that is currently breaking backups for users who updated to these versions.
If your backups have been running in the past, and recently started failing without an obvious reason, then your MySQL server was probably just updated to one of these versions.
Error messages
You may see one of these errors when trying to back you your database
mysqldump: Couldn't execute 'FLUSH TABLES': Access denied; you need (at least one of) the RELOAD or FLUSH_TABLES privilege(s) for this operation (1227)
ERROR: The database dump process encountered errors, preferring to fail as a precaution (ED385)
mysqldump: Couldn't execute 'FLUSH TABLES WITH READ LOCK': Access denied for user
Temporary Solution
While this is patched by MySQL, one solution on your end is to grant the MySQL user you use to back up MySQL the needed permissions
GRANT RELOAD,PROCESS ON *.* TO 'backup_user'@'%';
FLUSH PRIVILEGES;
Note: ensure you replace backup_user by the actual user you use to back up your database.
After you do so, ensure that these permissions are applied by running:
SHOW GRANTS;
The bugs are actively discussed at https://bugs.mysql.com/bug.php?id=109685 and https://bugs.launchpad.net/ubuntu/+source/mysql-5.7/+bug/2003866
Temporary Solution — if you are using your own server to dump the database
If the temporary solution above still fails, and you are using your own server to dump the database, you may need to just use an earlier version of mysqldump — you could try the following:
mkdir -p /opt/mysql
cd /opt/mysql
curl https://cdn.mysql.com/Downloads/MySQL-8.0/mysql-8.0.30-linux-glibc2.17-x86_64-minimal.tar.xz | tar -xvJ
mv mysql-8.0.30-linux-glibc2.17-x86_64-minimal 8.0.30
sudo ln -s /opt/mysql/8.0.30/bin/mysql /usr/local/bin/mysql
sudo ln -s /opt/mysql/8.0.30/bin/mysqldump /usr/local/bin/mysqldump
sudo ln -s /opt/mysql/8.0.30/bin/mysqlbinlog /usr/local/bin/mysqlbinlog
This will install MySQL 8.0.30 on your machine, and will create symlinks to ensure that it is used instead of any other version.