Can't clean duplicate entries

If I try to clean up the list of duplicate entries I always get the answer:

    Computer locked. Please retry in a few minutes

All the entries I wanted to merge are still there. In the apache error log I find this:

PHP Warning:  Invalid argument supplied for foreach() in /usr/share/ocsinventory-reports/ocsreports/require/function_computers.php on line 250, referer: http://servername.domain.local/ocsreports/index.php?function=admin_double

I already updated the server (VM with CentOS Linux release 7.3.1611) from 2.3 to the latest version 2.3.1 because of this problem but the error stays the same.