Document doesn't save after signing. Error suggesting file is read-only and it is not!

When a user is trying to apply a signature to a document which has come from an external source and is located on a network share they get the following error after applying a signature.

"There was an error when attempting to commit this signature. The document was not saved.

The file may be read-only, or another user may have it open. Please save teh document with a different name or in a different folder. "

This doesn't happen with internal documents in the same folder.

And also if the external document is processed on a local directory it does not happen either.

This only started happening in the last couple of weeks.

This user processes quite a few documents of this nature so would like to fix rather than just dealing with a work around.

Acrobat v 10

Chris