Hi. I've had constant trouble with the blur tool as well, but noting advice from 2019 on this thread, I have disabled the Hardware acceleration and it now seems to work. But over 4 years ago there was an anticipation this would be solved and it clearly doesn't seem to have been.
I am running an M2 16Gb using MacOs Sonoma 14.1. and Affinity 2.4.0
The problem seemed to happen on any photo I tried to work with, but in this latest case it is a .jpg 17.9MP - 9.7Mb
I would appreciate news when this bug is resolved. Thanks
