check.cgi
doesn't escape the URL before the validation. It's causing an issue when the URL contains a '+' for instance.
Updating line 255 from
params = feedvalidator.validateURL(url, firstOccurrenceOnly=1, wantRawData=1)
to
params = feedvalidator.validateURL(escapeURL(url), firstOccurrenceOnly=1, wantRawData=1)
fixes the issue