Cell references have stopped working in my Excel sheets when they are
enclosed in parentheses such as in Sumif or Countif statements as in:

=SUMIF(L11:O11,"c11")

The reference to cell C11 remains lowercase and the program can't find the
referenced cell C11.

This is something that has just started as of this morning and no longer
work, even on existing spreadsheets that have made use of them for months.