This happens with exo 0.3.101 and with current trunk. Thunar, xfce4-panel, mousepad, terminal, xfce4-weather do not open their help pages correctly because calling exo-open --launch WebBrowser "file_uri" adds some extra quotes around the uri. Thus, the web browser tries to open 'file_uri', which of course does not exist.
I'm seeing this here as well.
*** Bug 5549 has been marked as a duplicate of this bug. ***
Fixed in revision 30251, it only quotes when there are spaces in the argument (so when '/some/patch with spaces/bla' is send).
*** Bug 5579 has been marked as a duplicate of this bug. ***