Problem with AD connection via LDAP for some users

Please help us by providing the following info. Before posting please also check the pinned "Known issues" threads and make sure that you're running the latest available version for your oC release:

Steps to reproduce
1.some users can not login
2."sudo -u www-data php occ ldap:search name" can not be resolved
3.user is not shown in OC user list
4.but - username test in the admin frond-end works like expected

Expected behaviour
1. all user should be able to connect
2. ldap:search should find them

Actual behaviour
Some users can not longer connect
A second server with the same configuration (different site, same AD) works fine.

Server configuration
Operating system: Ubuntu 14.04.1
Web server:apache2 2.4.7-1ubuntu4.13
Database:mysql-server 5.5.50-0ubuntu0.14.04.1
PHP version:php5 5.5.9+dfsg-1ubuntu4.19
ownCloud version (see ownCloud admin page):ownCloud 9.1.0 (stable)
Updated from an older ownCloud or fresh install:Updated since version 7
ownCloud log (data/owncloud.log, see
ongoing errors like -
{"reqId":"qcskWoc\/MN3mBIePtmLv","remoteAddr":"","app":"no app in context","message":"Error getting storage info for mount at \/Ebert\/files\/PowerPoint_Templates\/","level":3,"time":"2016-08-25T10:25:27+00:00","method":"PROPFIND","url":"\/remote.php\/webdav\/","user":"Schmitt"}
but for users that are able to login, maybe not related

Special configuration (external storage, external authentication, reverse proxy, server-side-encryption):

Integrity status for oC9+
No errors have been found.