the following is an abbreviated excerpt from the mkvtoolnix debug log. the exact same thing happened at the exact same point in transcode as well as dvdrip. This indicated a high ordr proability of breakage by missing file in Boost.


version:1
:debug:clean Starting logging for mkvtoolnix @58.0.0_0+qtgui
:debug:sysinfo macOS 10.15.7 (darwin/19.6.0) arch i386
:debug:sysinfo MacPorts 2.7.1
:debug:sysinfo Xcode 12.3
:debug:sysinfo SDK 10.15
:debug:sysinfo MACOSX_DEPLOYMENT_TARGET: 10.15
:msg:clean --->  Computing dependencies for mkvtoolnix:info:clean .:debug:clean mkvtoolnix has no conflicts


:info:configure checking for boostlib >= 1.66.0... yes
:info:configure checking for boost/multiprecision/cpp_int.hpp... no
:info:configure configure: error: Boost's multi-precision library is required but wasn't found
:info:configure Command failed:  cd "/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_multimedia_mkvtoolnix/mkvtoolnix/work/mkvtoolnix-58.0.0" && ./configure --prefix=/opt/local --mandir=/opt/local/share/man --with-boost=/opt/local/libexec/boost/1.76 --with-extra-libs=/opt/local/lib --with-extra-includes=/opt/local/include --with-xsltproc=/opt/local/bin/xsltproc --with-docbook-xsl-root=/opt/local/share/xsl/docbook-xsl-nons --with-po4a=/opt/local/bin/po4a --with-po4a-translate=/opt/local/bin/po4a-translate --disable-update-check --enable-qt --with-qmake=/opt/local/libexec/qt5/bin/qmake --with-moc=/opt/local/libexec/qt5/bin/moc --with-uic=/opt/local/libexec/qt5/bin/uic --with-rcc=/opt/local/libexec/qt5/bin/rcc 
:info:configure Exit code: 1
:error:configure Failed to configure mkvtoolnix: consult /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_multimedia_mkvtoolnix/mkvtoolnix/work/mkvtoolnix-58.0.0/config.log
:error:configure Failed to configure mkvtoolnix: configure failure: command execution failed
:debug:configure Error code: NONE
:debug:configure Backtrace: configure failure: command execution failed
:debug:configure     while executing
:debug:configure "$procedure $targetname"
:error:configure See /opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_multimedia_mkvtoolnix/mkvtoolnix/main.log for details.