No download window iOS 14.6

Downloading from a self-hosted Docker instance (latest image, digest: d3fc8c3dff8bc74ebd057e319993d35a7bac83b626e4635133215806b92aa9ea) does not work on iOS 14.6. The console error is:
Not allowed to navigate top frame to data URL.

Interestingly enough, it works fine on send.vis.ee. The instance is hosted behind Cloudflare. However, it does not seem to make a difference if the proxy function of CF is disabled or not. Any ideas what the problem could be?