We are sorry to hear that you have such issue. The 'html' argument is valid. We tested this method in our Cloud API console (https://api.groupdocs.com/v2.0/spec/#!/document/GetDocumentContent_GET) with GUID of HTML document and it works well for us (it doesn't return the 400 error).
The last week we had new release for Cloud, and it's possible that there was some issues, so we want to reproduce the issue you have and we need your help to reproduce the issue. We will continue to test your problem, it will be very helpful if you could share with us GUID of the document which you use.
Also you can test the method via our Cloud Api console (https://api.groupdocs.com/v2.0/spec/#!/document/GetDocumentContent_GET) and share with us results if there will be error.
Here you can see what types of formats you can use for obtain content using this method - http://groupdocs.com/docs/display/GDconversion/Conversion+Variants .
Hi Pavel, thanks for getting back to me.
Thank you for your patience. We fixed the issue and now all works well for us. Could you please check if it’s work well for you too and notify us if all ok.
If you will have more questions please feel free to contact us.