Can't Get Impex to Run
 

I uploaded all the files to my development folder and the Import box appears in the cp. When I click on Import* it asks for my customer number. I enter the number that is displayed in the Customer Area and immediately get an error that flashes on the screen for a second:

"Warning: Cannot modify header information - headers already sent by (output started at /home/xxxxx/public_html/forums_vb/impex/ImpExConfig.php:1) in /home/xxxxx/public_html/forums_vb/impex/impex_auth.php on line 23"

Then the page returns to the box asking again for my customer number. I have tried it in two different installations and get the same error. What's going on?

(vB 3.84 PL1)