Issue making amendments & posting, and changing settings

I seem to have a bit of an issue with 3 different installs of WordPress, each with a separate database but all on the same hosting.

They are:
http://www.daddacool.co.uk
http://www.beingamummy.co.uk
http://doitanyway.co.uk (part of a multi site install and uses a default theme)

I can add a new post but:
It won’t let me save the post, it dumps me back to the front page of my blog;
It won’t let me add images (I can select & insert and image from the media library but it doesn’t appear in the post).

In addition to this I’ve also got an issue with amending plugin settings:
I can install & activate new plugins but when I try to alter their settings, it dumps me back to my home page. I can add new plugins but can’t delete them.

I’m also getting a 400 error from Jetpack.

What I’ve done:
Deactivated all the plugins and cleared my browser cache- no change
Changed the name of my plugin folder to plugins.hold, flushed my browser cache- no change
Done both of the above via a rdp to another machine, to make sure it’s not a browser security issue- no change.
Checked the folder permission via cpanel’s file manager- all seem normal
Switched the theme back to 2015, cleared the browser cache and tried again- no change
Switched the theme back to 2015, deactivated the plugins via plugins.hold and cleared the browser cache- no change

I’ve looked at the error log for the last day and there were a lot of PHP “Notice: Undefined variable” errors at the time everything stopped working properly but since then, I’ve only had a string of Jetpack errors.

I’ve contacted my hosting company, Dataflame, who assure me there is no issue with their end (like they all do) but I’m getting confused. I do have plugins used by all three sites- updraftplus, akismet, yoast, w3 total cache and yoast seo, but if I’ve deactivated them properly, I can’t see how that could be an issue. If the sites were shared on one install or WordPress or all used one database, I’d be more inclined to think it was a WordPress issue but while they’re all hosted on the same package, they have different directories, different databases and different installs of wordpress.

Anyone got any ideas? They would be most appreciated!