I'm maintain my own stylesheet and it has been working just fine with the demo server. Since a couple of weeks I have been getting "Cannot open URL" even though it works locally (PlantUML Server 20200319-1304 / version 1202004 ) just fine.
For improved debugging in the future, maybe you can print the message of the IOException in the picture?
Since I don't want to publish my style sheet, maybe somebody can take a look at what exactly happens during rendering. I have tried to host my file on gitlab.com as well as gitlab.io so I'm quite confident it's not your IP being blocked :P