I have Comodo CA cert already installed
I did it by copy and paste into texboxes:
1. SSL Request - content of filename: AddTrustExternalCARoot.crt
2. SSL Certificate - content of filename: my.domain.crt
3. SSL Bundle - content of filename: COMODOHigh-AssuranceSecureServerCA.crt
Have you created the csr for this certificate in this ispconfig website? If not, then the ssl cert is incomplete as the key file is missing. To fix this, you will have to install the key in the ssl folder manually in the file my.domain.crt and then enable the ssl cert in ispconfig again.