Error while relaying tile - what does it mean?

[knilde]

Thanks for that really nice peace of work!

I use it to fill up a simple nginx-cache, that buffers tiles from OpenTopoMap.
(OpenTopoMap is slooooow, so the cache makey my life easier).
For that I modified main.js and added the caches URL as tile-server.

While downloading with MapTilesDownloader many errors occur. Most of them are "Error while relaying tile".
What exactly this error means?