Okay, the easiest approach I think would be to create a new text file with this content:
Save this file as
jabref.reg and try to open it with a double click.
Note: Windows will probably warn you before execution this file. It should not do any harm (just adding the value "group/Keyword/Separator"="," to your registry) - but I can not guarantee this.
After executing this fix JabRef will hopefully startup again.