I am now seeing the same problem.  The orange upload status bar never completes, the green Finished confirmation screen never appears.  I end the page session and reopen piwigo.  If it was a single photo upload, when I go to the album it did in fact upload it.  When uploading multiple files, some might get uploaded and the rest do not.

Where should we start looking?

http://www.f-106deltadart.com/piwigo/
Piwigo version

        Piwigo 2.8.3

Environment

        Operating system: Linux
        PHP: 5.6.27 (Show info) [2016-12-09 20:57:37]
        MySQL: 5.6.32-78.1-log [2016-12-09 20:57:37]
        Graphics Library: ImageMagick 6.7.9-10

Database

        10573 photos (first photo added on Friday 4 November 2011)
        196 albums including 3 physical and 193 virtual (12090 associations)
        367 tags (4394 associations)
        28 users
        1 group
        3452 comments
        148 rates