works again. fiddled around thanks to this thread
I first tried these two commands, to disable the apt cache compression.
echo 'Acquire::GzipIndexes "false";' > /etc/apt/apt.conf.d/98dietpi-uncompressed
/boot/dietpi/func/dietpi-set_software apt-cache clean
but probably this command by itself to clean the cache, would have been sufficient.
/boot/dietpi/func/dietpi-set_software apt-cache clean