NextCloud 27.0.0. Use system cron service to call the cron.php file every 5 minutes. Recommended for all instances. The cron.php needs to be executed by the system user "www-data".

Above is the entire warning message in the Administrative – Overview section of NextCloud.

[Solved] Change from Cron Service to Ajax

Select the correct radio button within nextcloud administration.

This eliminates the entire Cron problem without any additional configuration.

The Ajax service runs the job every time a user accesses a page of NextCloud.