I am running Dropbox under Arch Linux on a Hetzner server in a datacenter in Finland. Ocasionally I do data processing on the server because it's a fast machine. Sometimes uploads of these changes to Dropbox get stuck. Three days ago, the upload of a single 3.5 MB file took more than half an hour. There was almost no load on the machine, and it has a 1 GBit/s connection. It was just sitting there, and this is not the first time:

Speedtest to a server in Germany:

What could be the reason? How do I find out more?
I contacted support, but that was not helpful. It seems that there is no proper support for headless Linux installations with only text terminal access.