This line...

https://github.com/NginxProxyManager/nginx-proxy-manager/blame/develop/docker/rootfs/etc/nginx/conf.d/include/proxy.conf#L2

I believe should be...

proxy_set_header Host $proxy_host;

(I got the below to work by doing it)

Without that, proxy requests are being forwarded to websites with the original Host header and those sites are returning an error or a 404. For example, trying proxying data.example.com to an S3 bucket like mybucket.s3-website-us-east-1.amazonaws.com:80

S3 will reply with something like..

404 Not Found
Code: NoSuchBucket
Message: The specified bucket does not exist
BucketName: data.example.com
RequestId: J8K24Y80A04JNFY4
HostId: ksf8GNLOX46prb58795rY6bxvr81Lfm6xzKnWTJcXLEaomW69xYWmBJBvcWV4VMZJEZohq4CF4s=

Further, you cannot override this proxy_set_header command by adding it to the advanced custom config in the UI because it simply appends the value to the existing header. ie, S3 will reply as blow (note the BucketName)...

404 Not Found
Code: NoSuchBucket
Message: The specified bucket does not exist
BucketName: data.example.com mybucket.s3-website-us-east-1.amazonaws.com

Nor have I found a way to "unset" proxy_set_header Host. I tried using "Headers More" to no avail: https://www.nginx.com/resources/wiki/modules/headers_more/