The asp.net 3.5 will work once you enable asp.net version 2 from Plesk. To enable asp.net version 2 from Plesk please follow the following steps :-
1) Login to your Plesk Control Panel.
2) Click on domain name on which you want to activate the asp.net 2.
3) Click on "Web hosting " setting in the Plesk Control Panel under that domain.
4) Click on the checkbox saying "Microsoft ASP support" and then select the asp.net
version "2.0.xxxx"
5) Click on "OK" once you have enabled the asp.net from the above steps.
6) You may now check as asp.net 3.5 will work now for your domain.
