Downloads over HTTP stop after a small amount of time

Hi,
first of all i hope I am asking the question in the right category.
I have a container with the zlepper/solder image running. Everything works fine except for one user. If he tries to download a file from the website, it starts to download but fails after a short amount of time because of a time-out. Downloads from other servers are working fine.
After some investigation we found out that the issue might be between the server and docker/container. Requests to the server directly are working as expected. Maybe it has something to do with the Dual Stack Lite connection of the user? Has anybody an idea to fix this?