On this blog and others I manage, yes, the wpLicense is breaking my feed. In fact, that plugin has been a total pain in the ass to me for a long time, which has been a great disappointment because I want to include the Creative Commons License on my blog and in my feed.
There is no doubt that the problem is the wp-License plugin. By turning off all my plugins, then turning them on again one by one, I was able to isolate the problem to what the plugin tries to include when the feed is generate. Actually, it’s probably more that wp-License is not playing well with another plugin (one that also includes something in the feed when it’s built), but that shouldn’t make a difference.
When wp-License attempts to include the Creative Commons information in the feed it isn’t including a line break or space, which makes two lines of code run-on together (as best as I can determine). I wish I copied the validation error, but it took me long enough to find the source of the problem, I’m not going to break my feed again just so I can copy an error message.