503 on accessing calendars from one client

I have added my calendars (hosted on my own nextcloud server, running with the calendar app) in lightning on a new laptop, but they don’t work. Looking at nginx’s (in front of my nextcloud) access.log, I can see that a PROPFIND request (one for each calendar) reaches the server and is answered with a 503, I haven’t found any other traces of what happens. The calendars work fine from other clients.

What can be wrong?