not experieced it, but Ross-Tech wiki states:
17963/P1555 - Boost Pressure Control: Upper Limit Exceeded
Possible Symptoms
Reduced power output
Limp mode
Possible Causes
Boost Pressure too High
Hoses/Pipes incorrect connected, disconnected, blocked or leaking
Charger Pressure Control defective
VNT (variable nozzle turbo): nozzles stuck
Solenoid Valve for Boost Pressure Control (N75) defective
Possible Solutions
Check Hoses/Pipes to/between Components
Check Solenoid Valve for Boost Pressure Control (N75)
Check / Clean / Replace Charge Pressure Control
Check / Clean mechanism for variable nozzles