On Windows, the following characters are not allowed in a file name:

/ \ : * ? " < > |

with one of these characters you either get an error message by the windows system when you try to save a file or - in the case of "?" - no error message occurs but nothing is saved anyway until you remove the offending character.

/ \ : are already replaced since T7848: Kleopatra: Remove whitespace from suggested export filename, please replace the others, too, with underscores.