Hello. All the Citroen C5 with AdBlue have the issue where the tank would lose pressure due to the pump defect. I have spoken to someone from PSA and there is an out of warranty replacement program for the first tank replacement. The tank is about 1000 euros but PSA covers up to 70% of the cost. The issue seems to be applicable to newer models of C4 with AdBlue. If someone has issues or errors related to AdBlue, make sure to report it to the dealership and have if replaced.
Hope this helps.