> The luatex code contains the lines (in unistring.w)
>
> if (val == 0xFFFD)
>         utf_error();
>     return (val);
>
> in a function str2uni. I didn't really try to understand the code
> but it looks as if 0xFFFD is used as "invalid marker":

Interesting.  This is not actually correct, U+FFFD is a valid Unicode character; it would be better to use U+FFFE or U+FFFF for that.

Note that U+FFFD is the recommended character to use when a character can't be recognised while converting to Unicode from another encoding, so its presence is usually a sign that something went wrong upstream, but I assume Manfred is aware of that.

> The comment in the code says
>
> /* the 5- and 6-byte UTF-8 sequences generate integers
>
> that are outside of the valid UCS range, and therefore
>
> unsupported
>          */

That's correct, the longest valid UTF-8 sequence is 4 bytes.

Best,

Arthur