Sometimes very simple formulas like
=IF([Column1]<=[Column2], "OK", "Not OK") can result in the unhelpful “The formula contains a syntax error or is not supported” error message, even though the formula in the example is perfectly valid according to the documentation, it still wont accept it.
The solution is in the comma
It turns out that a SharePoint sites regional setting changes the meaning of the comma (,) in the formula syntax. The solution is simple (if you know it): Just use a semi colon instead (;).
The example above should then be changed to
=IF([Column1]<=[Column2]; "OK"; "Not OK") and everything is OK.