Hi Gwilso,
I don't see anything wrong in your rewrite rules.
I see a lot of false positives on 4XX errors on my campaigns. Sometimes the MozBot crawler comes up with some interesting URLs.
You have any indication this is a recurring problem? I'd check the log files to determine that.
I'd also correlate what you're seeing in SEOmoz Crawl Diagnostics with your Google Webmaster Tools crawl reports - see if Google is hitting this same URL.
Bottom line is this is most likely not worth fretting over unless you see a lot of log activity around this URL request. Crawlers do weird things sometimes. Doesn't necessarily mean your site is "broken" or that this is impeding your SEO or user experience in any way.
-Mike