This year I had flown on AC more (international and transborder). Almost every time I travel, even though I was able to check-in on-line, I was stopped at the gate. The agent would make a call, and had to clear me.
Today I flew from SFO. On-line check-in 24 hours prior was working well. Went to the gate to surrender my I-94 and changed my FFP number from AC to LH. The agent had no problem.
But when it's time to board the plane, the gate checker could not let me board. It shows DHP in the computer.
The gate agent made the call to certain number. After sometimes, she said everything is okay. She let me boarded the plane.
So, what is DHP? How to deal with future travel on AC to ensure no more DHP remarks in the computer?
Thanks...

Note: When I am on UA transborder or international, I don't have this problem. Same on LH, no problem.