Error 400 using download link from embedded code

terryz Thanks for sharing that, though unfortunately I don't think that we'd be able to determine the issue from that; a 400 like this should generally indicate that there was something incorrect about the HTTP request, but the request data would be encrypted in that capture so we couldn't read it anyway.

Perhaps instead we can try to approach this another way. Can you reproduce the issue with another client where you can inspect the actual requests that get sent? For example, I just tried to replicate the request you're sending using curl with a test shared link of my own, but it worked successfully:

curl -v --tlsv1.2 --http1.1 -X GET -L "https://www.dropbox.com/s/9kkiqvfe457hrs0/test.csv?dl=1" -o out.csv \
	-H "Host: www.dropbox.com" \
	-H "User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.124 Safari/537.36" \
	-H "Accept: */*" \
	-H "Referer: https://www.dropbox.com/" \
	-H "Accept-Encoding: gzip, deflate, br" \
	-H "Connection: keep-alive" 

(I'm guessing you're using HTTP 1.1, but that --http1.1 option can be changed to --http2 to match if you are using HTTP 2.)

Also, you could try your code with my test shared link to see if your code works with that. If it does, it would indicate an issue with the particular shared link you're using and not the code itself.

By the way, I see you're mimicking a web browser by sending those "User-Agent" and "Referer" headers; that may not necessarily cause issues, but I can't guarantee it. Further, given that those aren't necessary, it may be good to remove them.