WordPress throws this error when it receives data that does not meet the requirements for a site option. The core functions that update network‑wide settings run a validator that checks the option name, the serializability of the value, and the presence of required fields. If any check fails, a WP_Error object with the code invalid_site_option is returned.
The validator also ensures the call occurs in a proper multisite context and that the database schema matches expectations. When validation fails, the error appears in the debug log and may surface as an admin notice or a fatal PHP error if the WP_Error object is not handled.