Is the document protected? You can find out by opening the document's properties (Cmd-D) and then selecting the Security tab. Are there any limitations listed?
I do not think the document is protected. On the tab you suggest, there is Protection method: "no protection". The only limitations are: Document changes and Document arrangement which are both set to "not allowed". Other options (including "Signing") are set to "allowed".
Do you already have a signature field in the document, or are you trying to add one? How exactly are you adding your signature?
We do the following:
1. From menyu above the open document we click "Sign digitally". A blue field appears on top of the document to be positioned to a desired place - we do so.
2. A pop-up window "Sign with digital ID" opens with our "Digital identifier of a key chain" option selceted. We click "Continue".
3. Another pop-up "Sign as OUR NAME" with a date-time stamp opens. We leave "Lock the document after signing" unselected and click "Sign".
4. A prompt for saving the "signed" documnt opens and when we click "Save", the initially mentioned error appears.
NOTE: The document has already been signed digitally by our counterparty. We need to add our signature and send it back to them.
Additiona information: When we try the same procedure in a Windows PC now (with our digital signature designated for the PC platform) everything goes fine and it is possible to sign the document there. But we need to be able to sign the document on a certain date on our Mac laptop when we are away from our office.
We have solved the issue. It was found out that the certificate in the Mac key chain was incorrect. After replacing it, everything worked fine. Thank you for your assistance, anyway.