that Serbia was sending it soldiers to fight in Bosnia and Croatia

Croatia was also sending it's soldiers to fight in Bosnia. Albanians were also fighting in Bosnia. Arab Mujahideens were also fighting in Bosnia and Kosovo.

You can't say that Serbs have ever had problems there before!

No kidding. Only 600 000 Serbs were murdered in Croat run death camps during WWII. With another 500 000+ sent fleeing to non HDZ territory. The same insignia used by Ustashi in WWII appeared again in the 1990's - on the Croat flag, it's police and army uniforms. That, coupled with Tudjman's anti-Serb rhetoric was clearly enough to bring back some nasty memories.

there were other Serbs coming to live there because they were offered Croatian houses

No. The documented flow of refugees was to Serbia - as of the early 1990's. Where would these new occupants have come from?

other nations that live in Serbia never thought of doing what some Serbs did to Croatia and Bosnia.

Funny you mention that, because if you take a look at the breakdown of ethic minorities in the Balkans, you'll find that the most multi-ethnic country today is... well, SERBIA.

They were even sending them buses of hookers from Belgrade to have some fun!

Busses of hookers. I see.