I finally decided to update my old Winamp 5.8.3660 installation, with which I haven't had a problem with in years, to WACUP after testing it out and finding it to be stable and updated with nifty new features.
A very strange thing happened today. I updated my video card drivers (Core i7-7700HQ/Intel HD Graphics 630, NVideo GeForce GTX 1050 Ti) and restarted my PC, after which, WACUP failed to load!!! I tried reinstalling WACUP and the crash persisted. I tinkered around a bit and discovered it was a subset of the input plugins that were causing the crash, including in_mp3 and in_mp4. in_flac was the only one that still seemed to work. I then tried reverting to my old Winamp 5.8.3660 installation, and it worked perfectly.
This is very distressing.