This issue also occurs by using nomacs from official repo (also nomacs is an image viewer which relies on qt5-base package), so is not an AUR problem: I remove the AUR tag: if @Aragorn disagree, can set again such tag.
Since the stable update of 2020-10-01 [Stable Update] 2020-10-01 - Kernels, Mesa, KDE-Framework, Deepin, Systemd, Pamac, Xorg-Server, Qt
I am no longer able to play gif images on applications which relies on qt5-base package, which, in facts, provides
An example: on my system I’m using qimgv as image viewer, which is qt5 based application; since the update of qt5-base package to 5.15.1-1 when I launch it from a terminal window by feeding it with any gif image, I got the error reported as title:
[Error] QMovie: "Unable to read image data".
All is fine with image viewers which doesn’t rely on qt5.
There could be some workaround, as delete some config files of qt?