Can't append in a Dataflow | No errors indicated in the Dataflow's Status report

Hi everyone,

I wanted to append queries in my Dataflow in order to have a table with all the data, so far, logical.

Below is another screenshot of my final table : 

When I load all tables and don't append, it saves and refreshes normally. But if I do not accept the loading of the tables to append them into a single table and I load this one as above, then it saves but throws an error after 30 seconds, with an empty error message !

FYI, each table is exactly identical in architecture, with these 5 columns, the same type on each table etc.

I don't know what else to do since I know that the problem comes from the 'append queries' but I doubt it's a bug or a technical impossibility... any idea?

Thanks for your feedback !