This has been answered several times already and is fixed in b2evolution 6.7.4.

I have 6.7.4 and have issues. Firstly it is not displaying the content, only the title.

it also produce warnings

I notice that it also does not show the full post in single view? I am using Bootstrap Blog

Please double check that your 6.7.4 upgrade worked correctly and that your browsing isn't serving you a cached page for some reason.

I did follow those on install and NEVER enable page caching.
I checked for page caching at:
/admin.php?ctrl=collections&tab=blog_settings&blog=2
/admin.php?ctrl=collections&tab=site_settings
I also cleared all caches

the post content is not showing in _single.disp.php
and not even when changing the skin

Solved: /admin.php?ctrl=widgets&blog=2

I needed to enable "Item Content" in single container.

Also, Item Preview is also fixed, in fact it never had a bug (6.7.4) I accidentally mixed 6.7.3 files with 6.7.4 files

@achillis @fplanque

Preview worked pretty fine until I switched from http to https
Now, preview dos not work.

How to fix this?

Regards, Will

I figured it out.
I intended to switch only few of my collections from http: to https: (as matter of costs). Therefore, I supposed, I should keep the base_url in basic_config to http.
I tried some interventions and at least I changed also the base_url to https:
Now preview works pretty fine again.

And those subdomains I kept http do work without any problems, too.

I hope to find some time slots to make a how to post for those who intend to switch to https, too (because of GDPR). The procedure is not really difficult, but it needed time to deal with some obstacles, especially for me, who is not familiar with SSL and how things are linked on b2e. I suppose there may be more guys out there who find themselves in a similar situation.