Forgive me but that’s not really a good answer. I’m currently having the same issue, I have 4 wp installs on 3 different servers. 2 on one server and one each. Initially (yesterday), it was working fine on the server with multi instances but today that stopped working. Also, I thought it was a theme issue as I was fiddling with a new theme config (a couple of WP resets) and that’s when it happened.
I’m not sure what exactly the reset did (either WordPress Reset or Reset WP) but I’m almost certain it was what preceded my problem.
I’ve tried a couple reinstalls with the deletion of everything, including plugin disables, removal.
The issue here is that this also affected the theme i’m setting up, compare .htaccess and other dot files with other servers’ and nada, zip, zilch!
Any ideas?
