You receive “cannot complete this action” error message whenever you try to do anything on your site

This situation basically occurs when you try to backup restore or migrate the sites between two server farms, usually the biggest problem you may face while migration site between two farms is that the user SID gets Screwed for good and then the hell breaks loose when you cannot get users log in to the site the permissions are messed up,

Check these few things before your land up to a conclusion.

Symptoms Faced:
Cannot backup restore or Delete a site collection, you face “Cannot complete this action” error message from Both GUI and Command prompt (I mean stsadm, of course)

This error message sounds a permission problem to you so there you go … start checking these

Run the command stsadm -o migrateuser with the problem account, if its successful you are done otherwise try

stsadm -o updatefarmcredentials

Remember you will have to hit IIS reset after this

Check if the account is available on WSS_WPG; WSS_ADMIN_WPG; IIS_ADMIN_WPG

If the migrate user command fails because you use the same accounts both times then create a temporary account that you create in AD and run the command as

Stsadm –o migrateuser –oldlogin domain\problem account –new login domain\temporary account –ignoresidhistory
Then run
Stsadm –o migrateuser –oldlogin domain\ temporary account –new login domain\ problem account –ignoresidhistory

Make some other account as a site collection administrator, remove the problem account from site collection admin and re-add it
From the link http://susandoreydesigns.com/software/AboutSharePoint.pdf

When permissions are established by NT/XP network ID, problems can arise when an ID is stopped and restarted. When a network user ID is (re)established and there are pre-existing permissions for that ID in a SharePoint site by virtue of membership in site groups by user name (as opposed to domain group membership) and/or as the owner of a site, the person will not be able to access the site. Instead they will get an error message like “Cannot complete this action. Please try again”.
In this case the ID must be removed from the site. This may be easier said than done.

􀂃 Change the site owner to somebody else. This is done by the server administrator on page “siteusrs.aspx.”

􀂃 Remove the ID as a site user by removing it from membership in site groups, including any custom list permissions.

This is complicated because SharePoint does not use the text of the user’s ID but a SID, which is different each time the network ID is re-established.

· Finally when you are convinced that this is not a permission problem run the hot fix 956994, which can be obtained from the Support professional also you can try running a very generic KB 936867, which is included in Service pack 1 for MOSS
This fixes the issue for the site with broken permissions problems