Do you have any custom validation on a property within the block you are creating, please see this following :
See following link contians the solution
The exact status code may be different and your configuration might be changing the status code to 500.
If, this is not related, please also check the following link too
I am not sure about the exact issue but it might be related to Access rights?
Because when you create a block using contentArea create button it will use the "For This Page"/"For This Block" so make sure you have proper permission for these folders.\
Could you please mention the Episerver version here?
We have been experiencing some issues with creating blocks by clicking the "create"-link on content areas. The create-form is displayed, but when clicking the create-button we get an exception in the console.
When trying to create the block in the assets panel and draging it to the content area it works fine.
An upgrade to the latest Episerver version didn't fix the issue unfortunately.