Hello Russell, could you still see this issue?
Stack trace of thread 120123:
#0 0x00007f49c3823ce1 n/a (/lib/x86_64-linux-gnu/libc-2.31.so (deleted) + 0x3bce1)
Were there more lines following in the "Stack trace of thread 120123"? And as libc-2.31 is shown as deleted, I assume this happened while libc package got updated while the kwin-x11 process was still using the previous version. Kind regards, Bernhard