Golf 5 1.6 FSI Fuel Problems in Heat

Hello,
I have a customer's vehicle here and I'm not able to pinpoint the source of the problem.
Golf 5 1.6 FSI (Engine Code: wird nachgereicht) has problems while driving in hot weather. It starts with stuttering and can even cause the vehicle to stall and not start again.
That evening, as it cooled down, things improved somewhat, but there was still a slight stammering.

Error Storage:

Wednesday, 02, July, 2014, 13:43:00:58542
VCDS Version: RKS 12.12.0

Address 01: Engine Electronics Label file: RKS\06A-906-033-BGU.lbl
ECU Part Number: 06A 906 033 CA
Part and/or Version: SIMOS 71 1.6l 2VG 9862
Encoding: 0000071
Operating Number: WSC 79235 146 83762
VCID: 60C996A8B3A70D1EF43-8035
1 Error found:

16555 - Fuel Measurement System (Bank 1)
P0171 - 006 - Lean System - Sporadisch

Readiness: 0000 0000


I have previously tested the fuel tank vent valve, and it operates and allows air to pass through even when the engine is running.
Activated carbon filter is not clogged, but the first one at 120,000 km is likely still functional.
All pipes up to the tank have been checked and blown through with compressed air.
On the tank itself, I checked 2 more lines for pressure and vacuum, and I didn't find any abnormalities.

If it's warm and the fuel tank has been recently filled, you'll hear a loud "hissing" sound when opening the fuel cap. I suspect that air is flowing in and not escaping.
The fuel tank ventilation valve should suck up the fuel vapors through the activated carbon filter due to the vacuum behind the DK. "Does this not also create a vacuum in the tank?"
How exactly is this being ventilated?

Maybe one of you can offer some advice.
Thank you.