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OK, thanks.
I had no idea that was related to it.

Had to roll back to 2.6.0 for Page Builder to be ok again.

Did that but no difference. I think this is actually one of the extensions, as I’ve rolled back to 2.6.4 and it started fine, then I happened to add the extensions back one at a time, and Page Builder broken it, and deactivating that fixed it.

I’m going to try and upgrade to 2.6.5 and then 2.6.6 in turn to see this remains the case and then add an issue to GitHub.

Getting the same thing, since running the latest patches on various plugins.
Using WP 4.6.1 and Unyson 2.6.6

Possibly, but I discovered I’d forgotten to add the rewrite rules to the site config file (IIS) and it started to work perfectly!

Um no, this is within WordPress – the ‘Update WrodPress – Connection information’ screen – “/wp-admin/update-core.php?action=do-core-upgrade”

I recall having to enable some PHP features or add some plugin when I set this up the first time.

Thanks! One I’d not seen before.
Wordpress has a standard format for its cookies?