Hi Michael,
My first guess is that this comes from a binary mismatch (C based components compiled with a different Tcl version).
Compile with debug enabled in a fresh build dir, such as e.g. with the following command:
sudo with_debug_flags=1 build_dir=/usr/local/ns-src \
bash install-ns.sh build
If you still see a crash, run nsd under gdb and show me the backtrace. If there is then still some problem, i will try to install somewhere a VM with RHEL 7.9.
all the best
-g
