First, you check whether "path/to/the/object" really is a path to an existing object. Maybe, the object was deleted and your object has not correctly synchronized with the catalog on deletion.
Checked. They were gone.
Another potential reason could be:
For historical reasons, a ZCatalog can use either physical paths or request/url based resolution. The latter is not safe when used with virtual hosting. Should you use the old way, then you might now use a different virtual host than when the object was indexed.
I think it using physical paths. In the error there are no traces of VHM. Although the site is doing with both Apache and Squid.
When you want to recover from the error (without understanding where the problem comes from), you could use the "Update catalog" from the "Advanced" tab. It will uncatalog any object that cannot be resolved. Alternatively, you can uncatalog the problematic object explicitly.
Well, after writing the post, I cleared every index and rebuild them. I didn't know I could Update it. Clearing + Reindexing worked. Somehow this "lost" object was affecting Plone's portal_factory: I could not create any new document of the folder that should have the lost object. Thanks, Manuel.