A conditional statement such as
C7: if ((c8-c7)<=0,"",(C8-C7)*24)
will leave a null in the cell if the condition is true.
If a later cell adds this null cell, the calculation is invalid.
C10: =C7+C9 =>> Results ins #Value!
This operation has been allowed in all previous releases.
A work around is to use
C7: if ((c8-c7)<=0,"0",(C8-C7)*24)
but this clutters what would otherwise be a clean spreadsheet.
I think this is part of our work to be more Excel compatible. "" is not a value but a string while "0" can be converted to 0. For example " 0" can't be converted too.
You have either the posibility to use 0 or "0".
*** This bug has been marked as a duplicate of bug 37860 ***