System Admin -> HTTPS Setup

Create CSR, used CSR on our internal CA, created certificate from it.
Have pbx.cert and ca.cert
Paste each into the relevant fields.
Browser refreshes back to the ‘instructions’ tab being active and nothing happens.

It should be easy, or at least possible, to install a certificate, so what am I doing wrong?
My guess is that the certificate format is incorrect, but as there are exactly no details at all (at least in the UI) of what format should be used, I’m not able to make an informed decision.

Searching the web and this forum yeilds all kind of things from very long ago, none of which seem relevant. Can someone tell me what is needed to make this work, and then could those instructions be added to the UI?