Quick repair for cracked wallpapered ceiling

The cracking would imply a drying problem. I would think that the cornice was primed improperly. If the primer was of cheap quality or the painter used a paint with a built in primer for drywall. The primer may have not effectively sealed the plaster. Then the drying of the paint would be effected, basically the plaster sucking out the moisture to quickly. this could cause the paint shrink and crack as it dried.

Answer to comment.

You have to fill the cracks. either by sanding them out or filling them. If sanding isn't an option try coating an area with a light coat of mud and after it drys lightly wipe it smooth with a damp cloth. Then repaint starting with a good sealer, kiltz would probably do.

A type of self leveling paint might work without all the mud filling, but It depends on the size of the cracks. I would sample a couple areas to see what works best.

There are paints that claim to have this ability. I would still start with a good primer/sealer before anything.

Cashmere

This is not an endorsement of the paint only an example of what to look for. I have never used it.

If the cracks have appeared recently and are still growing (in length and number) then this potentially indicates something serious.

Before you attempt any repair - which is perfectly possible using the method you outline - get someone in to have a look so see what might be causing them.

Have you put anything up in the crawlspace recently, or taken anything out?

Have you had an unusually wet or dry spell? This will change the ground conditions possibly causing the house to move which in turn could cause the cracking.