I get the message above (Attempt to insert record on page where this table is not allowed) when trying to create a new record from an extension.
Perhaps the creation of tables of this type is deactivated via Page-TS-Config? Via mod.web_list.allowedNewTables / deniedNewTables
it is possible to disable the creation of new tables.
You need to check each Page-TS-Config in the rootline or have a look at the info module.
Perhaps try to create a new "root" page and add your table there. If that works, it is a Page-TS-Config configuration:)
in the ext_table you need this: \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::allowTableOnStandardPages('XXXXXX');