Datamart errors with a single non-erroring dataflow

[EggyFanClub]

Hi, 

I see this issue a lot online with no clear solution. 

I made some changes to my dataflow, and I'm finding it difficult to push these changes to my datamart.

I have no clear error message:

Any advice on where to go from here? I run into this issue everytime I make a change but have found some work around (like deleting the contents of the datamart and re-uploading) but this time it always brings up this error.