An Unexpected Exception Occurred error message on PBI Desktop only

You can try using Format function and convert value to number and then you can do the sum. 

Conversion Count Numerator = 

VAR mindate = MIN('DateTable'[Date])
VAR maxdate = MAX('DateTable'[Date])

VAR tocount_oil = Format(
CALCULATE(
    COUNT('All Transactions'[TRANS_REF_NO]),
    REMOVEFILTERS('DateTable'[Month Date First], 'DateTable'[Date]),
    'All Transactions'[TRANS_TYPE] = "Quotation", 
    'All Transactions'[TRANS_DATE] >= mindate,
    'All Transactions'[TRANS_DATE] <= maxdate,
    'All Transactions'[TRANS_DATE_CLOSED] >= mindate,
    'All Transactions'[TRANS_DATE_CLOSED] <= maxdate,
    'All Transactions'[TRANS_STATUS] = "CLOSED WON",
    'All Transactions'[TRANS_DIVISION] = "Oil"
),"0")

VAR tocount_dust = Format(
CALCULATE(
    COUNT('All Transactions'[TRANS_REF_NO]),
    REMOVEFILTERS('DateTable'[Month Date First], 'DateTable'[Date]),
    'All Transactions'[TRANS_TYPE] = "Quotation", 
    'All Transactions'[TRANS_DATE] >= mindate,
    'All Transactions'[TRANS_DATE] <= maxdate,
    'All Transactions'[TRANS_DATE_CLOSED] >= mindate,
    'All Transactions'[TRANS_DATE_CLOSED] <= maxdate,
    'All Transactions'[TRANS_STATUS] = "CLOSED WON",
    'All Transactions'[TRANS_DIVISION] = "Dust"
),"0")

VAR tocount = tocount_oil + tocount_dust

RETURN tocount