You could try leaving the ring in the chest, and complete one or more of the other quests first. Maybe doing things in a different order will avoid whatever the problem is, or just being stronger and killing the opponents faster could make a difference. The next clue that triggers a summoning may give the same result, but it's worth a shot.
If the problem is something in the game rather than the save, a reinstall might help (or verify local files with the Steam version).
I thought Bellegar was suppose to summon more opponents when the timer ran out?
See Lar's post
here on how to send them an Xbox save.
With the PC version, 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).