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Speak to your host.  I believe this is caused by SNI technology at the server level which can cause https versions to revert to another site. SNI is when you can have SSL certificates on a shared IP.

This is best dealt with by your hosting provider.

I am a big fan of SiteGround, but that is a load of rubbish.

I recently (http://moz.com/community/q/less-tags-better-for-seo) started reviewing my category and tag policy, and things have been going very well.  I thought I would share what I have done:

1. Removed all tags from site
2. Added unique descriptions for each post for the category excerpt.
3. Only had the category description on the first page and use the description like a post to summarise and interlink to sub-categories or posts.  This keeps pages from slipping down the number of clicks until it can be reached, improving link juice distribution.  I also reduced the number of posts showing to 5, to allow more focus on the description (main part) of the category post.

To add the category description on the first category page only in Wordpress, you need to go to the category.php or archive.php and change:

to

The overall aim was to have a hierarchal resource contained in the category page description.  Whilst this is still a work in progress, you can see an example of what I am trying to achieve here:

https://www.besthostnews.com/web-hosting-tutorials/cpanel/

https://www.besthostnews.com/web-hosting-tutorials/cpanel/mail/

If you have any further tips and advice as I continue to implement this (with good results so far), please feel free.

Also, you can use the Visual Term Description Editor plugin to allow the wysiwyg editor for the category descriptions.