In fact the issue is not coused neither by Contact nor by MijoSEF but by your template. To solve it:
OPEN
templates/YOUR_TEMPLATE/html/com_contact/contact/default_form.php
FIND
<input type="hidden" name="view" value="contact" />
ADD BEFORE
<input type="hidden" name="option" value="com_contact" />
If you've a qualified template this should be added there but in some templates is missed, which is a must for SEF components.