Hello @aarongable,
Thank you for your kind help. I just downloaded the Android 5.1 stock ROM that was available from the Wiko support website and installed it on the phone....and I couldn't believe it, but now it's working! Both connecting to the mail server as well as opening the website in the OS browser now works fine.
So something must have been really broken with the android version on that phone and it had nothing to do with the issued certificates! Sorry for the inconvenience and thanks again for your prompt help!
Have a nice day!