To reproduce, just run it:

$ klystrack-plus
[WARNING] Loading surface failed: Unsupported image format
[WARNING] Loading surface failed: Unsupported image format
zsh: segmentation fault (core dumped) klystrack-plus

It opens a blank window after about a third of a second,
emits the warnings within the next third of a second,
and segfaults before the second is up.

Fresh installation, no config.
Happens on both Sway and i3.
Unsetting WAYLAND_DISPLAY (in Sway) changes nothing.
Unsetting DISPLAY stops the blank window from opening,
but it still crashes in completely the same way.

Running under gdb doesn’t appear to show anything useful (despite downloading a few hundred megabytes of debug info):

Starting program: /usr/bin/klystrack-plus
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
[New Thread 0x7ffff6dae6c0 (LWP 607651)]
[New Thread 0x7ffff1cba6c0 (LWP 607652)]
[New Thread 0x7ffff13ff6c0 (LWP 607653)]
[New Thread 0x7ffff0abd6c0 (LWP 607654)]
[New Thread 0x7fffe0bff6c0 (LWP 607655)]
[New Thread 0x7fffdbfff6c0 (LWP 607656)]
[New Thread 0x7fffdb7fe6c0 (LWP 607657)]
[Thread 0x7fffdb7fe6c0 (LWP 607657) exited]
[New Thread 0x7fffdb7fe6c0 (LWP 607658)]
[New Thread 0x7fffdadfd6c0 (LWP 607659)]
[New Thread 0x7fffda5fc6c0 (LWP 607660)]
[WARNING] Loading surface failed: Unsupported image format
[WARNING] Loading surface failed: Unsupported image format

Thread 1 "klystrack-plus" received signal SIGSEGV, Segmentation fault.
0x000055555559403c in ?? ()
(gdb) bt
#0 0x000055555559403c in ?? ()
#1 0x000055555557eeb7 in ?? ()
#2 0x0000555555563b24 in ?? ()
#3 0x00007ffff7a0f790 in __libc_start_call_main (main=main@entry=0x5555555639c0, argc=argc@entry=1, argv=argv@entry=0x7fffffffe0d8) at ../sysdeps/nptl/libc_start_call_main.h:58
#4 0x00007ffff7a0f84a in __libc_start_main_impl (main=0x5555555639c0, argc=1, argv=0x7fffffffe0d8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffe0c8) at ../csu/libc-start.c:360
#5 0x00005555555646e5 in ?? ()