"invalid UTF-8 character" in nie:copyright from image files (PNGs, JPEGs)
Both EXIF and IPTC seem to be very handwavy wrt the encoding of the strings contained, and this seems to be a trait of the libraries we use for that metadata too.
In result, and with the right circumstances (eg. image edited in a non-utf8 environment), we may get textual information containing non-utf8, eg:
nie:copyright "Bayerische Staatsgem�ldesammlungen, Sibylle Forster" ;
It seems we have to be more careful about the strings received there. FWIW this does not seem to be a problem with XMP.