Nope, never run across this before. Which version of Connections do you have installed? I’m thinking it is not the most recent judging from the URL. That “+” in the URL is very odd, Connections never created a query string like that when adding/editing an entry. So something must rewriting it and maybe that is causing the error.
Any way, the latest version of Connections no longer uses that URL; actually it uses internal actions. maybe updating would resolve the issue.