Edit any custom CB Activity auto actions you've created and resave them. There was a bug with their migration that's now fixed. Based off the below error line it appears to be happening in the CB Activity action.
Code:
17 cbautoactionsActionActivity->execute() /public_html/libraries/CBLib/CB/Legacy/cbPluginHandler.php:881
cbautoactionsActionActivity is the PHP class for CB Activity action. This migration bug is fixed in 6.3.7, but existing broken actions from previous builds have to be fixed manually by resaving them.