E.g. for File:Brewsteri nurk.xcf my ancient GIMP 2.8.14 reports "XCF error: unsupported XCF file version 11 encountered" while my brand new 2.10.2 opens it without problems. So my guess is that some software (ImageMagick???) needs to be updated. File:3d text Veniamin Bibikov fotolia-849x478-637x359.xcf is even better example – the first version was written by an old version of GIMP and two latest by 2.10.
$:acko\> convert T196054.xcf T196054.png convert: memory allocation failed `T196054.xcf' @ error/xcf.c/ReadXCFImage/1358. convert: no images defined `T196054.png' @ error/convert.c/ConvertImageCommand/3258. $:acko\> rpm -qf /usr/bin/convert ImageMagick-18.104.22.168-1.fc28.x86_64
Seems to be https://github.com/ImageMagick/ImageMagick/issues/1133 in upstream (which is not fixed yet)?