Hm, I find it frustrating that I found no posts about the issue... but think it might be a recently introduced bug, since according to https://line6.com/support/topic/46067-importing-irs the manual mentions:
"HX Edit updates the HX Stomp hardware's IR list automatically. HX Stomp can load and store up to 128 IRs at a time. 48kHz, 16-bit, mono, .WAV type IRs of up to 2,048 samples are natively supported; however, HX Edit lets you import IR .WAV files of different sample rate, bit depth, length and stereo format, and the app will convert these attributes automatically before sending to HX Stomp".
The file I uploaded doesn't match the format HX Stomps "stores" the file in at least, which means that the conversation mentioned above is either not happening or failing.
However, I found a way to recode it with `ffmpeg` (installed with `brew` on Mac OS):
ffmpeg -i input.wav -acodec pcm_s16le -ar 44100 output.wav
Once I tried loading `output.wav` as a the IR, it loaded the file without error.
Thanks!