Cohort Analysis Revenue

// original
Cohort Revenue =
VAR CohortStartMonth =
    SELECTEDVALUE( 'Transactions'[Cohort] )
VAR CurrentMonth =
    EOMONTH(
        CohortStartMonth,
        SELECTEDVALUE( 'Periods Out'[Value] )
    )
VAR TransactionsInCohortPeriod =
	// You should never attempt a filter over
	// a fact table involving RELATED. This kills
	// performance because RELATED uses
	// CALCULATE behind the scenes and this
	// in turn effects context transition which
	// makes your system die... as you've witnessed.
    FILTER(
        'Transactions',
        RELATED( 'Date'[EndOfMonth] ) = CurrentMonth
    )
VAR Invoices =
	// SUMMARIZE should never be used to do
	// any calculations in it. NEVER. There are myriads
	// of reasons but I don't have time to explain.
	// SUMMARIZE is safe only if you do grouping and
	// nothing else. On top of that, it looks like this
	// summarization is completely redundant since
	// later on you do a SUMX over the total column
	// anyway. Partitioning the table by PartnerID
	// is also totally useless in this case because of
	// the final summation.
    SUMMARIZE(
        TransactionsInCohortPeriod,
        'Transactions'[PartnerID],
        "Invoice Total",
            SUMX(
                'Transactions',
                Transactions[amount]
            )
    )
RETURN
    SUMX(
        Invoices,
        [Invoice Total]
    )


// Here's probably what you want:

[Cohort Revenue] =
VAR CurrentMonth =
    EOMONTH(
        SELECTEDVALUE( 'Transactions'[Cohort] ),
        SELECTEDVALUE( 'Periods Out'[Value] )
    )
VAR InvoicesTotal =
	calculate(
		sum( Transactions[amount] ),
		keepfilters( 
			'Date'[EndOfMonth] = CurrentMonth 
		)
	)
return
	InvoicesTotal