Thanks for the followup, @timstspry!
The way this failed seems to be a consequence of how Terraform’s plugin discovery is case-sensitive but Windows filesystems are case-insensitive: I think what happened here is that Terraform downloaded the same package twice, extracted both copies into the same directory (with one therefore overwriting the other), and then scanned the directory to make sure that both appeared. But both did not appear, because one overwrote the other one.
On the plus side, the forthcoming Terraform 0.13 release introduces a new provider naming scheme designed to accommodate third-party providers, and in the process of implementing that we’ve made the rules for provider names more explicit, including consistent handling of case. So the particular quirky failure you got here should not appear in Terraform 0.13, and it should either work or produce a more reasonable error message.