Problem with AD connection via LDAP for some users

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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 https://central.owncloud.org/t/how-to-find-webserver-or-oc-logfile-enable-php-logfile/808):
ongoing errors like -
{"reqId":"qcskWoc\/MN3mBIePtmLv","remoteAddr":"10.3.3.19","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):
nothing

Integrity status for oC9+
No errors have been found.