I couldn't wait this long .... ;)
It was a 6.8-current problem. Your hint with /usr/lib/libc++abi.so.3.0 was helpful:
when notecase startet it missed libc++abi.so.5
I did a whole new upgrade to -current and now it runs perfect!
I still will install it at home on a fresh 68-stable so that it matches your setup...
Ha, fine! Well done!
(the screenshot is from NotecasePro running on OpenBSD 6.8-current on an I3 window manager)
and it looks similar to the version running on my MAC: