The HR Department at a large university where I work has a PDF form that I cannot sign using Adobe Reader, DC, Acrobat Pro 8 and Acrobat Pro XI.
When I look under file > properties > security, it says that signing "is allowed".
But when I click on "I need to sign", all the choices are greyed out. Also work with certificates is all greyed out.
It's very similar to if I sign and lock a document, the "i need to sign" and "work with certificates" would be greyed out. But in that case, file > properties > security would show 'signing not allow".