Just installed ISPconfig 3.1 with install script on ubuntu 18
Installed IONcube
Ran softaculous installer, no issue
Click on link in ispconfig page (https://xx.xx.com/softaculous/): This site can’t provide a secure connection xx.xx.com sent an invalid response.
Try running Windows Network Diagnostics.
ERR_SSL_PROTOCOL_ERROR
"Click on link in ispconfig page (https://xx.xx.com/softaculous/): This site can’t provide a secure connection xx.xx.com sent an invalid response." - SSL is not installed on port 80 hence you are getting this error. Softaculous runs on port 80 hence please install the SSL on your domain at port 80 then you wont face the SSL error.
For white blank page in Softaculous refer the guide below: