Now there is a way to fix the problem with pamac.
I’ve been using a very small number of KDE apps on Xfce DE, and that caused some conflicts, especially with breeze, xcursor-breeze and antergos-xfce-meta. So some time ago I uninstalled antergos-xfce-meta because of the conflicts, but that seems to cause pamac freezing.
Now I uninstalled package breeze (with some related KDE packages) and reinstalled antergos-xfce-meta (no more conflicts), and pamac is working well again.
So, the lesson here is: one has to be very careful when using packages from another DE. Even though it may work well in the beginning, an update may (partially) break something, often because of changes in dependencies.