Unusual Behavior: Unable to preview drafts, wysiwyg broken, can’t disable plugin

I’ve been troubleshooting a site and been unable to clear up issues for a while now. Hoping for some help, as the solution seems like maybe it requires some “out of the box” thinking or advanced knowledge to resolve…

I have a site with several mysterious behaviors that I am not used to. The first one is my main focus on solving and I am including information on the other problems, only in case they are all related.

1. “Sorry, you are not allowed to preview drafts.” when previewing pages/posts

This goes for pages/posts that are already published or recently saved. ChromeTools tells me that there is a 403 (Forbidden) error when I attempt to preview. Note: all users are having this issue.

I have tried the following:
– Saved permalinks multiple times. I have tried switching permalink settings.
– Updating WP to the latest version.
– Disabling all plugins *exception, I cannot disable All-In-One WP Migration (see below)
– Setting default TwentySixteen to the active theme.
– Verifying that .htaccess file is correct. It is set to the default settings for a single site.
– Verified that the site URLs are set correctly in Settings -> General. (Both WordPress Address and Site Address are set to https://website.com which is what we want)
– Looking for anything unusual in the WP-Config file. The only thing that I found that was interesting, was define(‘RELOCATE’,true);. I tried commenting that out and it did not help.
– Clearing Local browser cache and cookies.
– Attempting a different browser.

2. Another problem: It is not possible to click the visual tab on the WYSIWYG Editor. It does nothing. This includes when we have special page editor plugins active (WPBakery for example), but also happens with the classic WP editor.

3. Another problem: The site is not accessible at all when we disable the WP All-In-One WP Migration plugin. So, this is the only plugin that I’ve had to leave enabled when troubleshooting.

If feels like to me that this could be some sort of strange configuration issue. But I have no idea. The site is running on Apache Linux and is an AWS EC2 instance.

Any advice or ideas for troubleshooting is super helpful, as these issues have been an annoyance for months.

Thank you! ??