Sidebars can't be placed next to the header area anymore

[RetoSca]

System Information:

CentOS 6.6
Parallels Plesk 12.0.18 Update #30
Parallels Presence Builder 12.0.7

Hello,

the feature "Move the sidebars to the top of the page and place them to the left or to the right of
the header area" doesn't work anymore. The sidebars are greyed out:


Does anyone know how we can fix this?

Thank you.

Best regards,
Reto