If you have upgraded MariaDB 10.5 to 10.6, you will see that Nextcloud 21 will stop working and you’ll see this message :

In the log, if enabled, you can see these errors :

"An exception occurred while executing a query: SQLSTATE[HY000]: General error: 4047 InnoDB refuses to write tables with ROW_FORMAT=COMPRESSED or KEY_BLOCK_SIZE."
"An exception occurred while executing a query: SQLSTATE[HY000]: General error: 1036 Table 'oc_preferences' is read only"

This because with MariaDB 10.6 the InnoDB’s COMPRESSED row format read-only by default (MDEV-23497).

For to resolve the problem enough set this parameter in the Mariadb config:

 innodb_read_only_compressed = OFF

After restart the database, Nextcloud 21 will start working again.