This problem hit me again. Last time it had resolved itself on it's own. So I tried to gander a guess as to why that was?
Then I figured "cookies".
I cleared the cookies for my domain (domain.com) and then reloaded the ISPConfig page, logged in again, and then opened Softaculous. Bang, it's working.
ps. not my actual domain. Was illustrating that I did domain.com instead of sub.domain.com which is the FQDN for the server (real FQDN not being given out). My browser wouldn't let me do sub.domain.com, so I cleared all cookies for domain.com instead.