Well as I wrote this is a isue.
Found the solution on vbulletin:
In IE6 the last character entered is hidden. This is only in QR and only in RTL. It does not affect any other browsers, editor boxes, or LTR languages.
Verified on default style with no modifications.
It can be fixed by making the following change....
In showthread_quickreply find two instances of
style="width:100%; height:{$editor_height}px"
Replace these both with
style="width:<if condition="!is_browser('ie')">100%<else />$stylevar[formwidth]</if>; height:{$editor_height}px"
It's only a temporary fix, but it works.
Works for me fine now
