Hi Natasha -
It does look like the issue is due to the format of that particular RSS feed at https://www.bis.org/list/bis/index.rss , although I do not specifically know why the StreamOnce RSS Connector tile is reporting that it is invalid.
I've done some testing on my side and I've been able to reproduce the same error you're seeing, and I've filed a new bug for the Jive engineering team to help better understand why this is happening. The bug report has been filed under ID STREAM-1807.
My assumption at this point, from looking at the rss feed source, is that it may be due to the rss feed being in an older format, which may explain why some other sites work and this one doesn't, although I would expect the StreamOnce RSS Connector to still work. The engineering team will need to investigate the actual cause.
If you would like to keep updated on the progress of the bug I'd encourage you to open a new support ticket around this issue and reference ID STREAM-1807. We can associate the case with the bug and once the bug is resolved the Support team will reach out to you in the case and let you know the details of when a fix will be expected, what version it will be in, etc.
Lastly, I've also gone ahead and moved the question to the Jive Training and Support Resources place so that others may chime in if they are seeing the same problem.