Most likely due to data corruption in your database. Typically this means there are versioned records of that page stored and no actual page type record. This can also happen with there is a node record and no document record. I'd run a query directly against the cms_tree and 'cms_document` tables to look for that record. Most likly missing in the document table. The solution would be to add a dummy record in the document table and link it to the node table. Then restart the website and attempt to delete the page again.