Welcome! This forum has a treasure trove of great info – Scouters helping Scouters! Just a heads up, though - all content, information, and opinions shared on this forum are those of the author, not the BSA.
For some reason, if I copy and paste the url into the field, I get the same result. But I found that the 1st character is a space character. When I removed that, it worked for me.
Do you have the Feature Assistant extension? A recent change to Scoutbook broke the extension’s recurring event features and one side effect of the breakage is the URL error. If you disable the extension before editing an event it should save ok.
I hope to have a workaround in the extension so events can at least be saved, but the recurring feature is likely to be dead until there is a Scoutbook change.