"IF" excel formula has Parse error

I input a formula in excel, and it gave me an error, when I open the application. "Error parsing expression"

My formula is : =IF(ISBLANK(C33);MAX(A18;MAX(A19;0));C33)

or =IF(ISBLANK(I13);MAX(A18;A19;0));I13)

This formula works in other reports, which I have created.

Could anyone help me with this?

BTW, A18 and A19 are formulas, too.

Thank you!