As Acrobat 8 is quite an older and unsupported version ,I am sorry however we would not be able to help you out with this.
We would request you switch to the latest Acrobat DC for a better experience.
Up until a few weeks ago I was able to use the program with no problems at all. Any ideas as to why I would suddenly be limited in how I can use the program?
Some certificate with which a signature was signed may have expired. I have not used Acrobat 8 for several years now and do not remember what the policy was then.
My signature certificate doesn't expire until 2017
I do not have Acrobat 8 for several years now and am not able to research the problem further. If your certificate is still valid, it could be some Acrobat 8 bug but there is no chance it will be addressed since Acrobat 8 is 2 version past EOL.