apparent conflict with headers

It doesn’t have to be a theme, can also be a template.

But you can stop the plugin from sending headers.

See the FAQ:

I get “Cannot modify header information – headers already sent” errors
This is possible if another plugin or your template sends out header information before this plugin does. You can deactivate and reactivate this plugin, it will try to load as the first plugin upon activation.

If this does not help you out deselect “Send headers when user is blocked”. This will no longer send headers but only display the block message. This however will mess up your website if you use caching software for your website.

thanks for the suggestion! I checked but no plugin is responsible, and to be honest I am not sure about the theme either…

…actually, I used a web proxy to simulate that I was in a different country. Such proxies add their own header to stick a banner at the top of the page; could that be the reason for the error? would you or someone try with vpnbook.com/webproxy and tell me if you get the same results?

I got either this:

The requested resource could not be loaded because the server returned an error:
403 Forbidden (?).

Or the page if the country was not blocked.