Same problem here. I am using Cloudflare too. Seeing Cloudflare mentioned a couple of times here I logged into it to check my settings and saw that the SSL encryption mode was set to “Flexible”. I changed that to “Full” and my posts now get published. Note: if it matters I had already had the “Always Use HTTPS” option enabled.
Agreed. Any way to get the previous version?
That All in one migration plugin looks good. I found this video showing how it works that might be useful to you.