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PHP Deprecated: strlen(): Passing null to parameter #1 ($string) of type string is deprecated in plugins/updraftplus/backup.php on line 2866
The function encrypt_file() define the variable:
$updraftplus->get_job_option('updraft_encryptionphrase');
In the get_job_option() calls UpdraftPlus_Options::get_updraft_option($opt), which could return null. But NULL is deprecated as #1 parameter for the strlen() function.
As i assume, you should not pass NULL to the strlen() function in backup.php:2866 file in option not exists.