IT/Software/Applications/Scanning/gscan2pdf
Ah. Easiest is to download the source for 2.6.4 and untar it with:
tar xvfJ gscan2pdf-2.6.4.tar.xz cd gscan2pdf-2.6.4
Download the patch (attached) into the same directory. Patch the source with:
patch -p1 < 0001-Fix-warning-message-about-pdftk-again-by-making-sure.patch
Remove the soft link you created from the flatpak.
Then start gscan2pdf from the source tree with:
PERL5LIB="blib:blib/arch:lib:$PERL5LIB" perl bin/gscan2pdf --log=log