What has been changed on your system before the problem occur?
Try to reset the Lightroom prederences -> Resetting Preferences in Lightroom CC/6 and the try again.
BTW: The lastest version of Lightroom 6 is version 6.14. you should install the lastest version because there are severeal bugfixes implemented.
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Metadata->Show Metadata for Target Photo Only should be unchecked.
Thanks a lot ! And by the way: why does it say "Show Metadata" and not "Assign Metadata" ?
why does it say "Show Metadata" and not "Assign Metadata" ?
That option controls both the display and assignment of metadata. When it's checked, and you've selected multiple photos, the Metadata panel will show the metadata from all of the photos.