Strange date separator in Excel
 

After a certain moment (maybe after installation of some office update?)
formatting of date values in all my excel workbooks changed. I use . (dot) as
the day-month-year separator, but now when I open any Excel workbook the dot
is replaced by some unprintable character displayed as a white rectangle. I
tried to change the formatting to dd.mm.yyyy, the separator is now OK, but
when pressing F2 the wite rectangle is there again. The problem is that I am
not able to enter any date value into Excel, when the white rectangles are
replaced by dots (or any other like dash, slash or space...), the date value
becomes text...
I have Windows XP (Slovak), Office Professional 2003 (English). The date
separator in my system is set to dot too.
Please help!