Toilet overflowing into shower when flushed what's wrong?

Hi gamago and welcome to The Plumbing Page at AskMeHelpDesk.com

[Toilet overflowing into shower when flushed what's wrong?
You have a blockage downstream from the shower that must be snaked put. This can be snaked from several locations.
1- Pull the toilet and snake from there,
2- Remove the shower strainer and snake from there. **OR**
3- snake from the lavatory roof vent, Be sure to put out enough snake to reach the base and 15 more feet, Good luck, Tom