Caused by: org.spongepowered.asm.mixin.throwables.ClassMetadataNotFoundException: com.aetherteam.aether.blockentity.AbstractAetherFurnaceBlockEntity

Are you sure this happens only with Deep Aether? If so, check if you have the latest versions of Aether and Deep Aether.

Going to close this issue since too little information is included. A crash report would be necessary.