Planes returned to base after dumping fuel due toilet issues
An aircraft with full lavatory waste tanks can not continue on it's flight as it is a requirement to have a specified number of serviceable toilets on every flight.  It is usually the responsibility of the Purser or In Charge Flight Attendant to check the waste tank volume when they prepare for each flight. On most aircraft there is a control panel with a display that shows the actual waste volume. 

Here are some possible reasons why the waste tanks became full:


They were not checked before the fight
They were not emptied on the ground
They malfunctioned and ran continuously 
The display on the waste control panel malfunctioned


In any case these tanks can not be emptied while airborne, and the only course of action is to land and have the tanks emptied or serviced. 

On a long haul flight like PVG to AMS the crew would not have been legal to continue their duty day after returning to PVG and the flight would have to be cancelled or delayed until a new rested crew could be found to operate the flight.