Error on startup

I don't see it on either:

Debian 10:
python3-pyqt5 5.11.3+dfsg-1+b3
libqscintilla2-qt5-13  2.10.4+dfsg-2.1 

MX-Linux 21 (Debian 11 spinoff)
python3-pyqt5  5.15.2+dfsg-3
libqscintilla2-qt5-15  2.11.6+dfsg-2 

This is MX Linux 19.4 per the recommendation of the guide that's floating around. I may upgrade/change anyway, I'm having other unrelated issues with this distro.

I tried my old MX19 developent machine as well and don't see the issue. It reports the same versions as Debian 10 as expected.
 

Can you open a terminal and enter;
python3

then
from PyQt5.Qsci import QsciScintilla

And see if you get the same error?