Get Logged Out of the Backend as Soon as I Click on a Menu Item?

[TDSA]

Hi Folks, 

We created a brand spanking new v 1.7.5.0 installation and have been working it for a week or so.

Everything has been running fine until a few hours ago when I got home and tried to login to the backend.

It logs in fine, but as soon as I click on any menu item on the LHS, (Like Orders or General or Module Manager etc.) it kicks me out and takes me back to the login screen. (Where you put in your l/p.)

I then click Login again as the login details are saved, I see the Dashboard and in the case of if I clicked on "Orders", I see the Orders Page,  but again as soon as I click on anything, it kicks me out.

Please help urgently as we need to go live on Monday and cannot do anything without being able to use the backend!

Something that may or may not be helpful: I also run a site that is still running Prestashop v 1.6.1.6. and I am able to login to the backend perfectly.

Edited February 16, 2019 by TDSA
Extra info. (see edit history)

[TDSA]

I have just tried to login again and it is now doing it with both backends. Both the Prestashop v.1.6 and the v1.7 websites?

Why on earth ill it be doing that?

Edited February 17, 2019 by TDSA
Spelling (see edit history)