I saw the same thing with Apple Mail on my iMac yesterday. All the browsers were fine with HTTPS on the same server, including Safari, but IMAPS gave bogus warnings about the R3 intermediate cert. Same cert as for HTTPS, but it was being rejected. I did reissue the cert and restarted the server, but it seemingly didn’t clear up until about an hour of fighting later. There seems to be a cache somewhere, because I can’t tie my last measure before it started working to any reason it should have done the trick.