TL/DR: user.ini file (which only your host should be able to get at) can screw you over
I started to move my core domain to a new host on Friday. I followed the usual procedure and blow me down what a problem I had getting /blog back up. Error messages all over the place. I deleted the database, removed the wordpress files, deleted the database user and basically I tried to reset everything. Except the problem(s) wouldn’t go away. Through various tinkering I could change the error message the site was putting out but I couldn’t get past the errors. I tried everything. Every single trick in the book. Eventually, after a long time of running into brick walls, I called tech support. Once they’d worked out what the problem was, they fixed it very sharply, but it took a while to get to the issue which was…
A corrupt user.ini file.