Cell M1 is a column header of a table (a proper Excel Table (called Tabelle1 in your file)). You're not allowed to use formulae in the header of such a table. Each header of such a table must contain something (cannot be blank). If it is blank, Excel assigns a name to it; Spalte1 in german, Column1 for an english locale. Try to delete any other column header and you'll get Spalte2 etc. Right-clicking the header (or any cell in a table column) and choosing Delete|Table Columns deletes the table column (cells to the right move to the left) and column M is no longer part of the table, so you can put what you like in M1.

I have no idea why, when you try to put a reference to another cell (eg. =W5) it returns plain value 0 (even if there is something else in W5). I can understand it removing the formula, but not why it puts in a 0 (if you try and do that again with another header it puts in 02 etc.).

If your .xls file uses macros you could write a macro to change the headers automatically.