Google calendar integration - gateway time-out

I’m trying to integrate google calendar.

I already had a google assistant integrated, through a specific google cloud project.
I tried to activate google calendar in this project + configured OAuth credentials.
But impossible to have it working in HA.

I now try with another google cloud project, only for google calendar. Did it, with OAuth credentials.
In HA I managed to build the credentials through google calendar integration request. But then, once created, with or without reboot, google calendar integration is not working. When I try to re-integrate it (without removing existing credentials), I get after around 1 minute delay the following message : “504 Gateway Time-out”. No token appears downloaded.
I never managed to go further, after severals days of multiples retries, with incremental unsuccessful changes.

My HA instance is running on a PI3, accessed through a reverse proxy (nginx) running on a PI4 in my network, using a duckdns https address. It all goes through a router which forwards to the reverse proxy ports 80, 81 and 443.
On the PI4 is also running a pi-hole instance, which manages DHCP and DNS for all my network.

Any help would be greatfully appreciated !
I suspect now something in my network configuration, but I can’t see clearly where to dig.

No idea ?
Nobody faced similar issue ?

Still blocked…

Solved ! The issue was indeed my DNS.
Pi-hole for a reason I don’t understand didn’t manage to serve the request issued by HA when trying to contact Google servers. I added a secondary DNS (Google one) in my router configuration, after Pi-hole. And it’s now ok.