AppEngine - Client closed keepalive connection

Я развернул свое приложение Symfony 4 на Google App Engine (гибкая среда).

Сборка вроде бы прошла, но загрузка приложения в Chrome приводит к ошибке:

Oops! An Error Occurred
The server returned a "500 Internal Server Error".
Something is broken. Please let us know what you were doing when this error occurred. We will fix it as soon as possible. Sorry for any inconvenience caused.

Журнал App Engine здесь:

 2019/02/02 04:33:13 [info] 9#9: *6 client 172.17.0.4 closed keepalive connection
 A 
 GET 404 191 B 0 ms Chrome 71 /favicon.ico A  GET 404 191 B 0 ms Chrome 71
 GET 500 930 B 6 ms Chrome 71 /connect/google A  GET 500 930 B 6 ms Chrome 71
 GET 307 316 B 6 ms Chrome 71 / A  GET 307 316 B 6 ms Chrome 71
 172.17.0.4 - - [02/Feb/2019:04:32:13 +0000] "GET /connect/google HTTP/1.1" 500 930 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36"
 A 
 172.17.0.4 - - [02/Feb/2019:04:32:13 +0000] "GET / HTTP/1.1" 307 316 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36"
 A 
 2019/02/02 04:29:08 [info] 9#9: *1 client 172.17.0.4 closed keepalive connection
 A 
 GET 500 930 B 6 ms Chrome 71 /connect/google A  GET 500 930 B 6 ms Chrome 71
 GET 307 316 B 5 ms Chrome 71 / A  GET 307 316 B 5 ms Chrome 71
 172.17.0.4 - - [02/Feb/2019:04:28:08 +0000] "GET /connect/google HTTP/1.1" 500 930 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36"
 A 
 172.17.0.4 - - [02/Feb/2019:04:28:08 +0000] "GET / HTTP/1.1" 307 316 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36"
 A 
 GET 404 191 B 10 ms Chrome 71 /favicon.ico A  GET 404 191 B 10 ms Chrome 71
 GET 500 930 B 207 ms Chrome 71 /connect/google A  GET 500 930 B 207 ms Chrome 71
 172.17.0.4 - - [02/Feb/2019:04:27:50 +0000] "GET /connect/google HTTP/1.1" 500 930 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36"
 A 
 GET 307 316 B 271 ms Chrome 71 / A  GET 307 316 B 271 ms Chrome 71
 172.17.0.4 - - [02/Feb/2019:04:27:49 +0000] "GET / HTTP/1.1" 307 316 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36"
 A 
A  2019-02-02 03:22:43,817 INFO success: nginx entered RUNNING state, process has stayed up for > than 1 seconds (startsecs)

A  2019-02-02 03:22:43,817 INFO success: php-fpm entered RUNNING state, process has stayed up for > than 1 seconds (startsecs)

A  [02-Feb-2019 03:22:42] NOTICE: ready to handle connections

A  [02-Feb-2019 03:22:42] NOTICE: fpm is running, pid 7

A  2019/02/02 03:22:42 [notice] 8#8: start worker process 9

A  2019/02/02 03:22:42 [notice] 8#8: start worker processes

A  2019/02/02 03:22:42 [notice] 8#8: getrlimit(RLIMIT_NOFILE): 65536:65536

A  2019/02/02 03:22:42 [notice] 8#8: OS: Linux 4.9.0-8-amd64

A  2019/02/02 03:22:42 [notice] 8#8: nginx/1.10.3 (Ubuntu)

A  2019/02/02 03:22:42 [notice] 8#8: using the "epoll" event method

A  2019-02-02 03:22:42,339 INFO spawned: 'nginx' with pid 8

A  2019-02-02 03:22:42,338 INFO spawned: 'php-fpm' with pid 7

A  2019-02-02 03:22:41,335 INFO supervisord started with pid 1

A  2019-02-02 03:22:41,329 WARN No file matches via include "/etc/supervisor/conf.d/*.conf"

A  2019-02-02 03:22:41,329 CRIT Supervisor running as root (no user in config file)

Я не понимаю эту ошибку. Как я могу ее решить?