Yesterday I tried to upload an XML dump to my site about 3 times, all of them failed. I tried again today, and I got the same error message:
Error 503 Backend fetch failed, forwarded for 2a02:587:b421:3700:3901:2c6d:1595:4f, 127.0.0.1 (Varnish XID 54593578) via cp11 at Sun, 21 Feb 2021 07:48:44 GMT.