Overview:

Using the non-standard ASCII character ł in the Key Message content results in an error in the Offline Veeva CRM Application: Incorrect Packaging Structure. This occurs once the Key Message is downloaded. 

Answer:

Replacing this character to a normal ASCII character resolves the error.

This behavior is working as designed, this character must not be used in the compressed (.zip) content.

Related Documentation:

CRM Help Documentation: Content Packaging