The open source mod called demo manager by UsAaR33 and AnthraX is responsible to load the demos, not RypelCam.
Here is a forum topic by the current developer AnthraX:
https://ut99.org/viewtopic.php?t=13567And his GitHub repo with the demo manager source:
https://github.com/stijn-volckaert/udemoThe changelog suggests this problem has been fixed:
[FIXED] Downloader for missing packages either crashing or not working
Should there still be problems with demo manager in that non-UT99-game, you could try to ask AnthraX if he'd help you out.