Latest update of Darktable (2.3.6.0-2) also pulled in a new openexr (3.0.5-2) and imath (3.1.1-1) which seems to have broken Luminance-HDR but also the EXR import plugin of Gimp.
Reverting/downgrading to openexr-3.0.5-1 and imath-3.0.5-1 restores sanity to Luminance-HDR and the Gimp EXR file loading plugin but kills Darktable 2.3.6.0.2
Downgrading Darktable to its previous version of 2.3.6.0-1 resolves that last problem as well. It seems this update may be slightly out of sync.