Infocus X1 dust problem, not on lens

I just moved to my new place, and i had to take my infocus of its mount (its been there 2 years). I cleaned the dust filters beside the lamp often, to be sure they werent clogged with cat hair and stuff.

Now i just mounted it back on the ceiling. The thing is, now there seems to be some kind of "stain" outside of the image while projecting in 16:9, and when i take my projector close to a wall, i can see dozens of white dots on the projection (see picture).

Now i used a compressed air can to try and remove the dust but it doesnt do any good. Is there something i can do except take it to the service center (and pay a hell of a lot more that i accept to pay for a 4 years old projector)?

i didnt open the projector box, or anything like that yet.

Thanks for any help you can give, this is about the only place i can find that i could find any answer.