Can't send a secured document for signature

I have a form with editable fields and the signature field already on it.

If I click on Fill & Sign, I get the option to send the document to a third party for signature which is great and what I need.

If, however I add security to the original document but allow printing and filling in form fields and signing existing signature fields, I lose the option to send the document out for signature. If I cick on Fill and Sign, the request signature option is greyed out.

If I click Request Signature button, the pop up asking for signers email etc appears, but when I click to select the file (the protected one), I get an error message saying 'Could not send / unsupported file type'!

No idea what's going on, but I need to be able to protect the documents before they are sent out.

Anyone got any ideas please?