I cannot import external themes

Hi all:

I'm trying to import external themes following the next steps:

0. Create a new File in the  Portal containing the *.zexp file generated 
by the theme exporting tool.

1. Add a new theme:

  ID: printable
  Theme URL: 
http://localhost:8081/webgen/workspaces/otrotheme/downloadFile/file/printable.zexp

2. Click on check for Updates. I get the "New theme available!" on the 
"Status" column.

3. Check my "printable" theme and click on "Install updates".

4. I get next error:

Error Type: 	ExportError
Error Value: 	

Invalid export header

Traceback: 	

Traceback (innermost last):

    * Module ZPublisher.Publish, line 115, in publish