Unable to Add/Edit new Test case Status code 400 is displayed.
The Support Team will work with you on the Support Case that has been created for this issue.
I know there is case created for this but I have seen this issue before. Have you tried using Incognito or Private browsing in your browser of choice? In the past the issue was caused by an extension or plugin stopping the process from being completed. If this works you will then need to eliminate the plugins/extensions 1x1 to determine which one is causing the issue.