"Cannot complete this action. Please try again."
Not a much telling error message from our microsoft friends!
If this error message appears when trying to access a SharePoint site, you have a problem with the user which you use to log in.
Try to delete and recreate the user in the system and take him into the portal again.
If you can't even access the site with the administrator (that was my case), you will have to create a new user account on the computer, for example "spsadmin", and set him as the SharePoint Farm Admin in the Sharepoint Central Administration Panel.