I have the same issue. I can't see anything in the logs that identify the root cause.
I see these allover lately but does not seem to break anything:
[Wed May 17 11:17:58.580171 2017] [:error] [pid 3372] [client 10.2.3.4:50247] PHP Warning: mysqli_connect(): Headers and client library minor version mismatch. Headers:50554 Library:50636 in /usr/share/ocsinventory-reports/ocsreports/require/function_commun.php on line 160, referer: http://ocs-server.lan/?function=admin_local_import