Sitecore EXM: Couldn't lock the item

Recently we encountered a warning message “Couldn’t lock the item LHSImage RHSCopy 2” when we changed an image in a custom email and clicked Save. We have several custom Sitecore EXM email templates, and most of them consist of a few custom Sitecore components. “LHSImage RHSCopy 2” is one of the components in an email template.

When an EXM user edits an image or text, they are actually editing one of these Sitecore components. Once they click Save, that component item gets locked to them. Later, if another user tries to edit the same component, they see the warning “Couldn’t lock the item…”.

A Sitecore admin can unlock the item.

Thanks
Munir