It seems that I’m not the only one having issues with the custom icons page. In Chrome, various extensions crash due to JS exceptions right after loading this page (e.g., Awesome Screenshots). Thereby, the loading tab crashes and the page is not shown. However, this only happens with a installed icon pack and Cloudflare Rocketloader. I’ve never seen such a behaviour till now.
If I load the page in the incognito mode (i.e., without extensions) and disable Rocketloader, there are no JS errors in the console. I suspect that there are parsing or compiling issues due to HTML, CSS, or JS bugs because the PHP executes completely. For instance, the W3C validator found 23 errors in the HTML code.
I noticed that after enabling Rocketloader the page shows for a short moment and then turns blank. The HTML code of the blank page consists of just a single Cloudflare script tag. That bears the suspicion on a DOM (i.e., JS <-> HTML) related issue.
Moreover, in contrast to the default icon set page, the preview and shortcode of the custom icon page doesn’t work at all (without JS errors, but the icon tag is shown). That seems to be CSS related. I’m going to write a separate comment about this issue.