Curl error 26 means the file couldn't be opened, which usually would happen when the file doesn't exist.
I believe the demos.tf plugin doesn't record demos but only uploads them, so if you don't have something like the recordSTV plugin that could explain it.
If you do have recordSTV or similar, it might be that the user you're running the server under doesn't have write permission to the folder the demos should be saved in and they're not getting saved. chmod +w would add that on linux, no clue if windows even has filesystem permissions.