Acrobat Pro XI recently updated to 11.0.15 and now forms cannot be signed, If they open in Reader DC they can be signed, and if I install a trial version of Acrobat DC it works. I have tried this on 3 different workstations. All of them worked, updated them and now none of them do. When you select add signature you can see your signature, you can drag and place it where you want to, but as soon as you left click to execute. the signature disappears. Again this does not happen in Reader DC or in Acrobat DC. Only in Acrobat XI 11.0.15
Could you please tell the OS you work on?
If you are windows,try to repair Acrobat once and see if that helps.
The two units I verified this on are both running Windows 7 Pro.
After encountering it on the first unit, I verified by using a different system not utilizing update 11.0 .15, that I could sign a document.
I then ran the 10.0.15 update and was unable to sign.
I uninstalled Acrobat XI and Installed the trial version of Acrobat DC and was able to sign the document again.
[signature deleted by host]