I had exported my data via the new “data migration” app that is available for nextcloud 24. A little bit more than 900 GB. Then I logged in to the webclient and downloaded the file, with firefox if that matters. At some point it stopped downloading and the nextcloud instance I was downloading from was showing random “internal server error 500”. Random means, that reloading the page would sometimes work, sometimes only for half the page, sometimes it would show the web server’s error message, sometimes the nextcloud error message.
I could not find the issue, rebooting did not help. I noticed with iotop that the mariadb process was writing 20-30 MB/s to the disk, even after rebooting. But the error log did not show any hints (at least to me).
The only thing that helped was to recreate the docker image.
I thought it was a one off and started to download the file again. Same issue after a few hundred GB. Again only recreating the docker image solved the issue.
So in case somebody else comes across the same issue, I hope this helps to “solve” it - and maybe that person is better at debugging it and can file an issue with useful information against the nextcloud server.