Hi,
This may happen in case the ACS database is corrupt. As a first step try rebooting the ACS appliance altogether, if that doesn't fix the problem, then provide me following information:
-ACS version that we are running. ACS GUI > System Configuration > Appliance upgrade status.
-The error message you get if any while installing the certificate.
-The certificate that you are installing. ACS will only work with 1024 bits key certificate.
-Try installing a self-signed certificate on the ACS and see if that gets installed. ACS GUI > System Configuration > ACS Certificate setup > Generate Self signed certificate.
Let me know if you have any questions.