Here's a list of known issues and solutions for Graphene 1.8. I'll be updating this post regularly, so keep an aye out.
1. Can't customise header title text
2. PHP sprintf warning
3. Featured image is not showing in slider
Simply re-save each of the posts/pages that are displayed in the slider.
4. PHP memory exhausted error
5. 500 Internal Server Error
I'm still investigating this. Join the discussion here: https://forum.graphene-theme.com/graphene-support/updated-to-18-500-internal-server-error
Remember, this is just a listing of known issues and solutions to make it easy for you to find the specific topics on them. Please don't create any new topic on any of the issues listed here, as several multiple topics all asking about the same thing will only hinder us from coming with a solution efficiently.
Please also don't post any question regarding these issues in this topic. Post in the issue's respective topics listed above instead.
Graphene 1.8.1 is now available, which contains the fixes to the above issues.