Changes to my datamart often fail on 400 error. I must say that error reporting in Datamart is rather poor, since all errors are very general. The desktop at least tells me what specific table faile...
I have had a session with your support and those Datamart credential settings are an unfixed bug, as of now.
However my "Enable load" problems were unrelated to those credentials.
In the end if I made only one change in my datamart at a time, it managed to save and refresh.
I did not get a confirmation, but in my case it looked as my 2 tables were simply too big (6000 and 11000 rows) and "enable load" on both of them at the same time ended up in an error.
In the end I have to admit I have dropped Datamart in favor of Dataflow. I am new to both, but Dataflow works much better for my case.