If you don't have a save before triggering the summoning (or if that is too far back to want to revert to), you will need to send your save to support@larian.com, with a description of the problem (DKS
support page).
Unfortunately they are on vacation next week, so there will be a delay until they can fix your save.
You can not find Saul again after freeing him. I don't know where he goes, but he doesn't stay in High Hall. The only way to successfully complete the Thorn in the Side quest and get a quest reward is to kill Saul.
In XP the saves are in the folder
C:\Documents and Settings\ %username% \Local Settings\Application Data\Divinity 2\Savegames
or in Vista / Win 7
C:\Users\ %username% \AppData\Local\Divinity 2\Savegames
You may need to set Windows Explorer to show hidden files and folders (
XP, Vista,
Win 7).
