Error -84 is EILSEQ: "illegal byte sequence". The kernel can return this signal, for example if it cannot decode its own binary in the region nearby the requested kprobe, or it finds instruction patterns that its decoder has not been taught to handle. It's not a systemtap bug.
To figure out the exact cause, we'd need to see the systemtap script, the address stap picks for the probe (see stap -p2 -v ...), and a (objdump -d) disassembly of the kernel in that region.