css is not found when redirecting mantis to separate folder

[Evadik]

Hi.

I have installed mantis in the 'http/mantisbt' folder on my hosting and created subdomain that was directed to this folder as following:

RewriteCond %{ENV:REDIRECT_MYFLAG} ^$
RewriteCond %{HTTP_HOST} ^mantis\.mydomain\.com$
RewriteRule ^(.+) %{HTTP_HOST}$1 [E=MYFLAG:1]
RewriteRule ^mantis\.mydomain\.com(.*) /mantis%{REQUEST_URI} [L]

When I open http://mantis.mydomain.com I get error:
"The requested URL /mantis/index.html was not found on this server."

And even when open http://mantis.mydomain.com/index.php - I get mantis opened but without css.

I investigated the problem and found out that mantis considers %path% as 'http://mantis.mydomain.com/mantis' which is not correct.

Please explain how to avoid this
"The requested URL /mantis/index.html was not found on this server."
and how to solve the problem of lost css.

Thanks.