As could be noticed from the error, Firebase path(key) doesn't allow certain special characters in the path.
While storing email as path, I recommend you to encode the email i.e., replace 'dots' from 'commas'. If you wan't to retrieve, you can decode.
public static String EncodeString(String string) {
return string.replace(".", ",");
}
public static String DecodeString(String string) {
return string.replace(",", ".");
}
Let me know, how it goes for you.
Update
Using UIDs to store user details is better than Email IDs.
