Sorry for the misunderstanding. Im pretty sure I did mention cloudflare in the first post: ... on my raspberry pi behind cloudflare and nginx proxy .... Might be a bit unnoticeable so sorry about that.
However disabling cloudflare did not fix it:
Im suspecting it might be the nginx proxy, like I said I configured it according to the solution on this post: "Nginx Proxy Manager" causing failure when uploading files.
Is there anything wrong with that?