Hi Zorzi
Hope you are doing well and sorry for the trouble. As described you are unable to sign the document and get the unknown error
Is this a behavior with a particular PDF file or with all the PDFs? Please try with a different PDF file and check.
What is the version of the Adobe Acrobat DC you are using? To check the version go to Help > About Acrobat and make sure you have the latest version 21.01.20149 installed. Go to Help > Check for Updates and reboot the computer once.
If it still doesn't work, please try to turn off the protected mode for testing (Win Only) Go to Edit > Preferences > Enhanced Security > Uncheck 'Enable protected mode at startup' turn off the protected mode and Uncheck 'Enhanced Security' > Click OK and reboot the computer.
Note: Please turn on the security after testing to avoid any security risk.
You may also try to sign the PDF online at https://documentcloud.adobe.com/link/home/ and see if that works for you.
Regards
Amal