Hi, need some help, i have 6 users connected to an omni via pppoe. One user cannot access some sites, and others can connect to those site but then again not to different sites.

Any ideas?

Wrong dns servers set at clients computers? Check if you can access those webs by their ip addresses.

this might be a MTU issue.
check from the client computers what maxium MTU size you can send over the link.
then adjust yout PPOE MTU size accordingly