No error messages are shown in Firefox, even with debug mode active. The same behavior (no fcktoolbar is shown).
Original config.php and config_defaults.php from 1.90+ have been overwriten from the same files from 1.90RC2. In fact, these files are systematically overwriten since v1.87 or before. We've only modified the typical variables (db password, etc...).
Variables may affect to fckeditor, in config_defaults.php has never been modified, and they are currently set to the original values. That's it...
$homeurl = "$rooturl/admin";
$fckeditordir = "$homeurl/scripts/fckeditor.266";
$fckeditexpandtoolbar = true;
One time we obtain a message error accesing .../admin/scripts/fckeditor.266/fckstyles.xml (no access privilegies). But we can't get this error again.
We've tested fckeditor distribution is correctly installed on the server and accesible, like other versions before, just try
encuestas.upv.es/admin/scripts/fckeditor.266/fckstyles.xml
We've also installed another 1.90+ from scratch on another server and the problem persists (no fckeditor toolbar is shown). I can provide access and admin password to this new instalation if you want to test anything online.