We apologize for the inconvenience caused, as per the description above, you are getting an error message "An Unknown error occurred" while saving a PDF file which you have signed, Is that correct?
Please check for any pending updates of Adobe Reader from help>check for updates, reboot the machine after installing any updates and see if this brings any difference.
Try repairing Adobe Reader from help>repari installation, reboot the machine and try again.
Have you tried saving another PDF file? Is it specific to one PDF file or with all the PDF files?
Also, check the document properties of the PDF. Open PDF and go to File menu > Properties > Security.Let us know the security settings on the PDF.
This generally happens in a corrupt PDF file. Please share the following piece of information so that we can assist you in a better manner:
Let us know how it goes and share your findings.