Experiencing RSS Dashboard Feed error only on one feed that is the correct feed

I’ve got the dashboard feed network activated and displaying feeds on site. There are 3 feeds; one internal to network and two external.

One of the external feeds, http://doterrablog.com/feed/ is generating the following error:

RSS Error: This XML document is invalid, likely due to invalid characters. XML error: Reserved XML Name at line 5, column 39

I have tested the feed and it is correct. The other external feed is working ok.