Hi there!
Every time I try to build pages and categories I get this message:
Building Category Pages . . .
Building Category: International
Subcategories: 0
Links: 0
CGI Error:
Message: invalid directory name: blablabla ... Reason: contains illegal characters.

Any ideas to fix this?

I am assuming that you did not add your categories via admin.cgi which checks for proper format of categories and links...the problem is that in one of your categories near International either contains spaces (which should be replaced with an underscore character _), accented characters, etc.

I would recommend using the Check Cat option in the admin.cgi to find the problem category, or edit your categories.db manually. Also, I would recommend downloading and installing the Non-English Mod, if you are planning on using non-latin/english accented characters in your category names.

Regards,

Eliot Lee