librephotos: Can't connect: 504 Gateway Time-out

I set up a brand new instance of librephotos, using docker-compose on Synology. I made some changes to the .env file (use port 3001 instead of 3000) and the process appears to go well (one warning: ssh key not set). When I try to log in to localhost:3001, I get a 504 Gateway Time-out returned.

The proxy log says the following:

2021-08-18 07:35:56	stderr	2021/08/18 07:35:56 [info] 29#29: *19 client 172.19.0.1 closed keepalive connection
2021-08-18 07:35:54	stdout	172.19.0.1 - - [18/Aug/2021:07:35:54 +0000] "GET /favicon.ico HTTP/1.1" 504 569 "http://192.168.2.201:3001/" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.131 Safari/537.36"
2021-08-18 07:35:54	stderr	2021/08/18 07:35:54 [error] 29#29: *19 upstream timed out (110: Connection timed out) while connecting to upstream, client: 172.19.0.1, server: , request: "GET /favicon.ico HTTP/1.1", upstream: "http://172.19.0.5:3000/favicon.ico", host: "192.168.2.201:3001", referrer: "http://192.168.2.201:3001/"
2021-08-18 07:34:53	stdout	172.19.0.1 - - [18/Aug/2021:07:34:53 +0000] "GET / HTTP/1.1" 504 569 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.131 Safari/537.36"
2021-08-18 07:34:53	stderr	2021/08/18 07:34:53 [error] 29#29: *19 upstream timed out (110: Connection timed out) while connecting to upstream, client: 172.19.0.1, server: , request: "GET / HTTP/1.1", upstream: "http://172.19.0.5:3000/", host: "192.168.2.201:3001"
2021-08-18 07:33:53	stdout	172.19.0.1 - - [18/Aug/2021:07:33:53 +0000] "GET / HTTP/1.1" 499 0 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.131 Safari/537.36"
2021-08-18 07:33:53	stderr	2021/08/18 07:33:53 [info] 29#29: *16 epoll_wait() reported that client prematurely closed connection, so upstream connection is closed too while connecting to upstream, client: 172.19.0.1, server: , request: "GET / HTTP/1.1", upstream: "http://172.19.0.5:3000/", host: "192.168.2.201:3001"
2021-08-18 07:33:51	stderr	2021/08/18 07:33:51 [info] 29#29: *17 client closed connection while waiting for request, client: 172.19.0.1, server: 0.0.0.0:80

What can be the cause?