I have exactly the same problem.
interwiki.local.conf:
sk https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solutionid={NAME}
I put 16x16 gif to the directory /usr/share/dokuwiki/lib/images/interwiki. It is accessible at the right URL:
http://mywiki.local/lib/images/interwiki/cp.gif but the page with my interwiki link shows the default interwiki icon instead:
http://mywiki.local/lib/images/interwiki.png.
The resulting HTML code looks correctly compared to the Wikipedia interwiki link:
<a href="http://en.wikipedia.org/wiki/Wiki" class="interwiki iw_wp" title="http://en.wikipedia.org/wiki/Wiki">Wiki</a>
<a href="https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solutionid=sk54061" class="interwiki iw_sk" title="https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solutionid=sk54061">sk54061</a>
So it seems that the definition of class="interwiki iw_sk" in some css is not updated?
Of course I cleared the cache both in the browser and in the DokuWiki:
rm /usr/share/dokuwiki/data/cache/?/?*.?*
Edit:
I checked the all the three CSSs linked to the page (for media all, screen, print) and there are styles for a.interwiki and a.iw_wp defined but a.iw_cp is missing. Should I do something to update the CSS?