debugging forms authentication problem

[z. f.]

Hi,

i have vb.net web application.
directory admin have web config that requires be in role Admin
upon first request it redirects to login page.
login page success and setts authentication cookie.
then redirects to original requested page.

the on_authenticate checkes the current.user to have instance but it is nothing
and authentication fails and framework redirects to login page again.

how can i debug my problem ?

i validated that the authentication cookie is sent to the server.

what else should i check and debug here?

please help it's urgent.

TIA, z.