Omega sidebar second shifts to the left after login

I have created a site using Omega with a copied HTML5 subtheme. My content zone layout :
Primary Layout=Normal, using 24 columns. Regions: sidebar-first is empty (1-column), content has content (16-columns), sidebar-second has other content (7-columns).

Everything went fine until I did css and js aggregation. Ever since the aggregation, the second bar always shifts to the left after login. Wiith no login and anonymous access, layouts are correct and as expected. I have done:
1. disabled aggregation for css and js (restored by backups before aggregation and the behaviour of shifting sidebar-second also existed, I just did not notice it earlier... too many browser windows open).
2. swapped the content of sidebar first and sidebar-second, i.e. sidebar-first has content, sidebar-second is empty. Layout was ok.
3. restored content to sidebar-second and emptied sidebar first. Sidebar-second has shifted to the left again. The shifting is one column to the left.
4. re-installed Omega theme
5. the problem exists across browsers (IE, Firefox, Chrome, Safari)
6. shifting also happens in Primary layout = Fluid

I have attached screenprints, I can also send url, with id/pwd to demo. Thanks for any help.

Abe