I have upgraded from 1.5.5f to 1.5.6a


I had my admin in a different folder named 'dif-admin', so I copied my existing admin 'dif-admin' to 'dif-admin_old'
All installed v156 a correctly.
I then copied the new 'zcadmin' contents to 'dif-admin' after installations.

I can see public site fine.
When I tried to login to admin I get "ERROR 0071: There appears to be a problem with the database. Maintenance is required."
My database prefixs are correct.

After renaming new install to 'dif-admin_new' then I switch back to my 'dif-admin_old' by after renaming 'dif-admin_old' to 'dif-admin' my admin works. Admin also shows that it is version v1.5.6a

Can anyone help to solve this please?