1071 Specified key was too long; max key length is 767 bytes

Usually there are two ways to fix this:

A) Configure your database server to support utf8mb4 (https://docs.nextcloud.com/server/latest/admin_manual/configuration_database/mysql_4byte_support.html).

B) If the database server does not support utf8mb4, disable it by setting 'mysql.utf8mb4' => false in config.php (emojis in filename, calendar events, etc. are not possible than and you have to accept the warning).

What happens for most people is: They change the maria/mysql configuration temporary to enable utf8mb4 but forget to write the changes to the configuration file. The next time a update is installed the sql server is restarted and the temporary configuration changes are lost. Some weeks later they try to upgrade Nextcloud and run into those utf8mb4 errors again.

Original report: #15058