druid who chooses spiritshift Stag can't cast spells in stag form, more often than not the character will freeze when the spell is supposed to be done. I don't know what causes the problem, it seems to happen randomly

In my experience, it's not random at all. Some spells cast every time, others never do. One can avoid casting those spells while spiritshifted, but it's quite an undue burden on the stag considering their carnage doesn't work correctly without the community patch.