cant view published posts or pages

I am using theme: wp-andreas01.2.0.
When i create a new post, or page, i can preview without problems. but when i publish the post or page, i get a http 404 error. The pages and posts are visible on my website but clicking them just gives me an error page.

I have used wordpress with other sites without problems, but with 2 sites i have just started, i get the same problems over and over:

https://www.minicrystalworld.com
https://www.furrypetstore.com

Is it the theme or is it something i am doing wrong?
Thank you for any help