Hello cakethat
We apologize for the inconvenience caused, as per the description above, you are not able to Fill&Sign a PDF form and save it using the Adobe Reader DC, Is that correct?
Please check for any pending updates of Adobe Reader DC from Help>Check for updates, reboot the machine and try signing the PDF file.
You can also sign the PDF file online through https://cloud.acrobat.com/fillsign and see if you get any error message. have you tried with any other PDF file and checked? Try with any other PDF file and see if the issue is reproducible.
You can also repair the installation files of Adobe Reader from Help>Repair installation. Reboot the machine.
Let us know how it goes and share your findings.
Regards,
Anand Sri.