I think you (the staff of owncloud) would like to know that there is a problem with you certificate chain.
You miss to pin the intermediate certificate (the Let's Encrypt one).
Most of the time nobody sees this because the browser download it for you and doesn't show any issue
but in certain cases it could be very annoying.
We use an web-proxy with DPI (deep packet inspection) to access the internet,
it is technique that is taking hold. It breaks the SSL encryption to analyze the traffic.
In this scenery the connection with your web server can't be establish until the client install the intermediate certificate on
his firewall or exclude the website from the proxy.
Here is the report of SSL Lab