If created a formula a while ago in which I refer to sheets with names
like 1 and 2 -- ROW($1:$2) this way I get a correct amount from the
sumproduct formula.

=SUMPRODUCT(SUMIF(INDIRECT("'" & ROW($1:$2) & "'!A2:Z1000"),
$A4,INDIRECT("'" & ROW($1:$2) & "'!B2:B1000")))

I want to rename the sheets to something which makes more sense but if
I try that (like for instance to sheet1) my formula returns an error?

Thanks in advance,

Mike