RSS feed parsing error in /base-category/category/feed/ (OK in /feed/)

I’m having difficulty troubleshooting this & hope some one may be able to point me in the right direction…

sitename.com/feed/
– OK

sitename.com/base-category/feed/
– OK (reverts to sitename.com/feed/)

sitename.com/base-category/category/feed/
– Does not validate
– XML parsing error: <unknown>:93:423: not well-formed

Can anyone give me any advice as to what this eliminates (in terms of where to look?).

I have tried;
– disabling all plugins
– changing theme to twentytwelve
– disabling all plugins & changing theme to twentytwelve
– fix-rss-feed plugin
– Fix My Feed RSS Repair
– manually searching for “blank lines” in php files

I’m not a developer & was hoping the discovery of the “category” issue may narrow down my search.

Any advice appreciated

Thanks