Note: The bug here is with code I wrote! This is related to
MDL-19274 about including tag information with blog posts.
The ->tagscheme field in RSS $item is a URL. If the URL contains an & symbol, this needs to be escaped in output, otherwise the RSS library generates invalid XML. My code forgot to escape it.
It is very unlikely that this would ever cause an actual problem in standard moodle install because the value used for tagscheme is always "$CFG->wwwroot/tag", so unless you have an & inside your wwwroot (which is quite difficult to achieve) there will not be a problem. However this causes an issue for oublog which uses a feed url, which might include &, for its tagscheme.
Since it is still a bug (albeit one very unlikely to occur) and since fixing it is harmless, I'm going to commit this simple fix to stable (after testing it with use of standard moodle blog).