A couple of things.
1) The Filename doesn't matter. Also .zip files work too. They don't have to be .tar.gz.
2) Make sure the folder structure remains intact with the subdirectories in the same place. This is most commonly a problem with images. The path on the page refers to images/file.jpg, but it's easy to zip a file without image in the image/ folder.
3) Uploading the error templates does not immidiately activate them. the old ones are still in memory. To activate them (in order of preference)
a) SSH to port 9092, use the GUI user/password, run the `reload templates` command. (this does not interrupt production)
b) restart the services: `service webwasher-csm restart` (this interrupts production)
c) reboot.(this clearly interrupts production)
Personally, I'd use the Language Pack Designer to easily create your own custom block pages. Email me and I'll I'll point you to where to download it.