Hi russz,
As per the issue description mentioned above, you are facing issue when trying to use fill and sign, is that correct?
Please try following troubleshooting steps:
1- Reboot the machine once if you haven't already and check if that helps.
2- If that doesn't work, check if an update is available for Acrobat using "Check for updates" option under "Help" menu, reboot the machine after installing update and try again.
3- -If issue still persists, try repairing Acrobat using "Repair installation" option under Help menu ( for Windows OS only), reboot the machine after repair and check.
Also, let us know dot version of Acrobat and the operating system installed on the machine? You may refer to the steps given in this link on how to check the version in Acrobat: Identify the product and its version for Acrobat and Reader DC
Let us know if you need any help.
Shivam