Web services error

[raunhar]

Presta Version: 1.6
I created a API using the web services. 
On running the script, the following error shows up. The API is being used by UniWare (india) for syncing stocks with Amazon and few other market places.

<error>
<code><!--[CDATA[18]]--></code>
<message><!--[CDATA[Invalid authentication key format]]--></message>
</error>

No changes were made to Presta Files.

Please suggest the solution.