Summarize virtual table

Bucause SUMX is an iterator function.

When you are writing measure should should always include the table name when referencing a column and never include the table name when referencing a measure.  It you measure it looks like [Minutes] is a measure.

Aggregate work per date = 
VAR tbl = 'Combine work per date'
RETURN
    SUMX ( tbl, [Minutes] )

Also, either of these measure should return the same data as the first one.

Aggregate work per date = SUMX ( 'Combine work per date', 'Combine work per date'[Minutes] )

Aggregate work per date = SUM ( 'Combine work per date'[Minutes] )