This error is likely to be caused by a corrupt website cookie or related to your browser cookies.  A quick test is try and open the site in Private Mode/Incognito and if it works, then the resolution below should work.

The resolution is to remove website cookies stored on your computer.

Depending on the type of system and browser version you’re running, the process is slightly different.

CHROME
Step 1: Go to the setting icon (top right of page) and then scroll down and select the Setting link.
Step 2: Scroll down and select “Show advanced setting..”
Step 3: Select the “Content Setting” button under Privacy
Step 4: Select the “All cookie and site data” button under Cookies
Step 5: In the search box, enter the domain that’s returning the Bad Request error
Step 6: Select the domain from the returned results and press the Remove all button

FIREFOX
Step 1: Go to the Firefox drop down menu (top left of screen, Firefox logo) then Options and then select Options.
Step 2: Select the Privacy TAB and then select Remove individual cookies
Step 3: In the search box, enter the domain that’s returning the Bad Request error.  
Select the domain from the returned results and press the Remove All Cookies button.