sorry for the late reply.
yes, that's the cause of your problem.
if you have a default page, you will face such problem.
please refer to it as "default page", because we call "default file mask" something slightly different.
i just tried right now, and found a possible workaround.
you must edit your search form (i guess in your template, but it may possibly be also in your default page).
(double-quotes not included)
after "<form "
you should add " action='~nodefault' "
and also you should enable Menu > Other options > Enable ~nodefault