Back on business - to be back on track, (things, events, situation etc ) being back to normal.
To me, back in business seems more "literal sense" (voltar aos negócios) or to state that things are working as planned.http://www.macmillandictionary.com/dict ... n-businesshttp://www.ldoceonline.com/dictionary/business
See "out of business"/"be in business"/be back in business.http://www.learnersdictionary.com/definition/business
Here the sense is similar to the one of ''back on business".
Sometimes people use them interchangeably. So don´t worry, you won´t be sued if you do.