When WHMCS is installed in a subdirectory of WordPress and you want to activate knowledgebase, announcements and/or downloads pretty permalinks, you need to make sure the .htaccess of WHMCS doesn't conflict with the .htaccess of WordPress.

Assuming that WHMCS is installed in a folder called whmcs, then update the .htaccess file in the whmcs subdirectory as follows (changes are in bold green).

RewriteEngine On
RewriteBase /whmcs
...