Just suggestion: I had similar issues with button displaying in EM before (4.6.x) (ie. Install ext button) and realised, that the button is available, but sprite image had wrong position which caused that the button was transparent,
Try to use ie FireBug to check it, and eventually fix the CSS (and report the bug)
Edit:
Some doom apparently, I just had identical situation just seconds ago ;) Fortunately found the reason: the ext list is displayed with ExtJS grid, which can contain custom filtering per column. Literally: when you for an example click on the column's dropdown arrow, you can in its filter set some value to narrow the search results.
Ie.: in the main filter field write news and additionally in the Extension Key column set other filter tt_news. (if you want to display only %tt_news% in result list of %news%)
In that case even if you'll clear the main filter, the one set on the column will stay active, so next time when you'll try to search for realurl in main filter, it will return empty list as Extension Key filter is still set to tt_news.
Weeeird, however helped me :)
You can recognize filtered columns as their names are written with cursive, finally you can just check every column and disable any filtering on each.