Any idea why this error appears sometimes?
I'm trying to reproduce it.
It happens rarely on file upload. The file is uploaded, but this error appears.
Then everything is ok.
Plone 4.3.10 (4313)
This can happen if you are running multiple Zope instances. Adding a file involves an HTTP POST to the add form URL followed by a redirect to a GET of the file's URL. If the load balancer sends these two requests to different instances, and the 2nd instance already had the folder in the ZODB cache, it's possible that the GET will be handled by the 2nd instance before it has received invalidations from the ZEO server to know that the database was updated with the new file.
The easiest workaround is probably to configure your load balancer to use sticky sessions so that it prefers to send the same client to the same Zope instance.
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