The incident happened Thursday afternoon shortly after the Airbus A319-100 took off from Lisbon’s Humberto Delgado airport for Paris Charles de Gaulle.
According to website Aviation Herald the aircraft was climbing out of Lisbon “when the crew stopped the climb, declaring PAN PAN, PAN PAN, PAN PAN reporting a hydraulic failure.”

Three calls of pan-pan are used in radiotelephone communications to signify that there is an urgency on board a boat, ship, aircraft, or other vehicle but that, for the time being at least, there is no immediate danger to anyone's life or to the vessel itself. This is referred to as a state of urgency.
It is different to a mayday call, which means that there is imminent danger to life or to the continued viability of the vessel itself.
Thus “pan-pan” informs potential rescuers (including emergency services and other craft in the area) that a safety problem exists whereas “mayday” will call upon them to drop all other activities and immediately initiate a rescue attempt.

“While working the related checklists the crew requested Lisbon tower to perform a runway check to determine whether there was any hydraulic fluid on the runway. The crew decided to return to Lisbon”, Aviation Herald states, adding “The runway inspection did not find any evidence of fluids.”
The aircraft landed safely back in Lisbon about 35 minutes after departure.

The plane remained on the ground in Lisbon for just over eight hours, then departed again and was estimated to reach Paris with a total delay of about nine hours.