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That’s … weird.
A fixed sidebar is one of the theme’s main features:
Can retrace your customization steps? Maybe you broke that at some point?
If you haven’t customized anything else, this problem may be a plugin conflict. Please attempt to deactivate all plugins. If the problem goes away, re-activate them one by one to identify the source of the problem.
Dear James. Thanks for your response.
I did reset all templates to origin. It works again. Thank you for your hint.