Argh. Just spent the better part of the day banging my head against the wall on this. Also watching eagerly for a fix that doesn’t require a downgrade.
Reckon this is the same issue being discussed at Relative paths to images in CSS doesn't work anymore (ie. background-image path)
…unfortunately, I did try the suggestion of just downgrading resolve-url-loader to ~2.3.1 doesn’t work for me, and chucks different errors:
Module build failed: ModuleBuildError: Module build failed: TypeError [ERR_INVALID_ARG_TYPE]: The "from" argument must be of type string. Received type undefined
…etc.