Invalid Application pool

Dear All,

We are from a webhosting company and we are using IIS 6.0 with Plesk control
panel. Here is the issue is that, In all the website's property we found that
the Application pool : has been filled with an error called <Invalid
Application Pool> (under Execute permissions in Home tab)

Also we checked the "Application Pools" ( Left pane in Inetsrv MMC ) where
all the pools are running fine and we are able to create and delete
application pools.

Could anybody know why it is ? and how to fix them. Because we are not
getting the proper Error codes from the web servers when we execute some
script like asp, php, aspx, pl... Instead just giving an error that "Internal
Server Error" . But the sites are running fine without any problem.

Please help. Thank you.