Cannot connect to server on my Ipad

[Captain Midnight 1937]

I have been having a problem logging onto only 1 web site recently, Safari advises " Cannot open page because it could not establish a secure connection to the server" using my I pad, however I have no problem connecting my I Phone to the web page using the same address. Please advise fix.
Don

 

[flaubert]

We probably need to know the models of iPhone and iPad involved, and also what versions of iOS are running on each of the two devices. I'd be curious to know the URL of the web site, if that isn't giving out too much personal information; then I could look at the certificate being used on the site, since that may provide a clue as to what is going on.