Some cursor sets don’t show the image of the cursor, but a white square. According to Stack Exchange, it’s because the symbolic link wouldn’t point to the correct image file. However, if I set the link manually, the white square persists. Any idea?
Try this:
gtk-update-icon-cache -f -t /path/to/cursor/theme/
gtk-update-icon-cache
You can also do this for regular GTK themes.
gtk-update-icon-cache -f -t /path/to/gtk/theme/
gtk-update-icon-cache
The KDE/Qt version of this is:
kbuildsycoca6 --noincremental
kbuildsycoca5 --noincremental
But the KDE/Qt version is not just about themes, and is usually not needed unless you’re having issues with .desktop files and mime types.

gtk-update-icon-cache -f -t /path/to/cursor/theme/
gtk-update-icon-cache
Didn’t work, … . Maybe it’s the cursor theme, other sets don’t have the problem.
Yes, very likely. Did you log and log back in just to be sure since trying my suggestion? Sometimes that may be needed.
But in general, themes not made by the DE can cause several types and degrees of issues at times. It mostly depends on whether the custom theme (whether GTK- or Qt-based) follows the rules of the design language properly, but can also be affected by Wayland and/or X11 compositors.
Confirmed error, despite restart etc.
Nevermind, there are other sets that work.