I was having this problem too.
However, I think I found the solution.
The ris template only has one space in each of the tags, e.g.
TY - JOUR
Looking at the ris file format on wikipedia, a RIS tag is supposed to have “two letters, two spaces and a hyphen”. So something like
TY[space][space]- JOUR
I also looked at the ris files I got from other publishers and they also have 2 spaces in the tag.
I’ve changed the ris.tpl file in my installation and found that it is now exporting .ris files that work fine with Endnote.
I would also recommend changing line 20 of the ris.tpl from
AU - {$author->given} {$author->family}
to
AU - {$author->family}, {$author->given}
This will just ensure that Endnote gives people the correct family name because some people have more than one part to their family name.