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Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 12.0.2): 15.0.5
Operating system and version (eg, Ubuntu 17.04): Ubuntu 18.04 + Docker
Apache or nginx version (eg, Apache 2.4.25): Nginx
PHP version (eg, 7.1): 7.2
The issue you are facing:
LDAP Users sometimes disappear from NC. The few times I’ve seen it, they’ve reappearred before I’ve been able to work out why they’d disappearred.
Is this the first time you’ve seen this error? (Y/N): N
Steps to replicate it:
- Have a working NC install.
- Wait for a user to say they cannot login
- search for the user in nextcloud, they’re gone.
- wait a day.
- the user is (usually) back
The output of your Nextcloud log in Admin > Logging:
{"reqId":"ZtpmRAg0idapW4vLDOja","level":2,"time":"2019-06-17T03:06:26+00:00","remoteAddr":"172.25.0.1","user":"--","app":"user_ldap","method":"GET","url":"\/ocs\/v2.php\/apps\/serverinfo\/api\/v1\/info?format=json","message":"Bind failed: 49: Invalid credentials","userAgent":"--","version":"15.0.5.3"}
{"reqId":"ZtpmRAg0idapW4vLDOja","level":2,"time":"2019-06-17T03:06:26+00:00","remoteAddr":"172.25.0.1","user":"--","app":"core","method":"GET","url":"\/ocs\/v2.php\/apps\/serverinfo\/api\/v1\/info?format=json","message":"Login failed: 'lfitt' (Remote IP: '172.25.0.1')","userAgent":"--","version":"15.0.5.3"}
{"reqId":"sqMeiVfKQgZxsI7viArF","level":2,"time":"2019-06-17T03:07:26+00:00","remoteAddr":"172.25.0.1","user":"--","app":"user_ldap","method":"GET","url":"\/ocs\/v2.php\/apps\/serverinfo\/api\/v1\/info?format=json","message":"Bind failed: 49: Invalid credentials","userAgent":"--","version":"15.0.5.3"}
{"reqId":"sqMeiVfKQgZxsI7viArF","level":2,"time":"2019-06-17T03:07:26+00:00","remoteAddr":"172.25.0.1","user":"--","app":"user_ldap","method":"GET","url":"\/ocs\/v2.php\/apps\/serverinfo\/api\/v1\/info?format=json","message":"Bind failed: 49: Invalid credentials","userAgent":"--","version":"15.0.5.3"}
{"reqId":"sqMeiVfKQgZxsI7viArF","level":2,"time":"2019-06-17T03:07:26+00:00","remoteAddr":"172.25.0.1","user":"--","app":"core","method":"GET","url":"\/ocs\/v2.php\/apps\/serverinfo\/api\/v1\/info?format=json","message":"Login failed: 'lfitt' (Remote IP: '172.25.0.1')","userAgent":"--","version":"15.0.5.3"}
{"reqId":"GgFZINmvyRvSZP4JxLuQ","level":2,"time":"2019-06-17T03:08:26+00:00","remoteAddr":"172.25.0.1","user":"--","app":"user_ldap","method":"GET","url":"\/ocs\/v2.php\/apps\/serverinfo\/api\/v1\/info?format=json","message":"Bind failed: 49: Invalid credentials","userAgent":"--","version":"15.0.5.3"}
{"reqId":"GgFZINmvyRvSZP4JxLuQ","level":2,"time":"2019-06-17T03:08:26+00:00","remoteAddr":"172.25.0.1","user":"--","app":"user_ldap","method":"GET","url":"\/ocs\/v2.php\/apps\/serverinfo\/api\/v1\/info?format=json","message":"Bind failed: 49: Invalid credentials","userAgent":"--","version":"15.0.5.3"}
{"reqId":"GgFZINmvyRvSZP4JxLuQ","level":2,"time":"2019-06-17T03:08:26+00:00","remoteAddr":"172.25.0.1","user":"--","app":"core","method":"GET","url":"\/ocs\/v2.php\/apps\/serverinfo\/api\/v1\/info?format=json","message":"Login failed: 'lfitt' (Remote IP: '172.25.0.1')","userAgent":"--","version":"15.0.5.3"}
{"reqId":"aihguTU9eTI2QhGaPXo6","level":2,"time":"2019-06-17T03:09:26+00:00","remoteAddr":"172.25.0.1","user":"--","app":"user_ldap","method":"GET","url":"\/ocs\/v2.php\/apps\/serverinfo\/api\/v1\/info?format=json","message":"Bind failed: 49: Invalid credentials","userAgent":"--","version":"15.0.5.3"}
{"reqId":"aihguTU9eTI2QhGaPXo6","level":2,"time":"2019-06-17T03:09:27+00:00","remoteAddr":"172.25.0.1","user":"--","app":"user_ldap","method":"GET","url":"\/ocs\/v2.php\/apps\/serverinfo\/api\/v1\/info?format=json","message":"Bind failed: 49: Invalid credentials","userAgent":"--","version":"15.0.5.3"}
{"reqId":"aihguTU9eTI2QhGaPXo6","level":2,"time":"2019-06-17T03:09:27+00:00","remoteAddr":"172.25.0.1","user":"--","app":"core","method":"GET","url":"\/ocs\/v2.php\/apps\/serverinfo\/api\/v1\/info?format=json","message":"Login failed: 'lfitt' (Remote IP: '172.25.0.1')","userAgent":"--","version":"15.0.5.3"}
{"reqId":"s0mNFYBZYVKFLw3R3E9K","level":2,"time":"2019-06-17T03:10:26+00:00","remoteAddr":"172.25.0.1","user":"--","app":"user_ldap","method":"GET","url":"\/ocs\/v2.php\/apps\/serverinfo\/api\/v1\/info?format=json","message":"Bind failed: 49: Invalid credentials","userAgent":"--","version":"15.0.5.3"}
{"reqId":"s0mNFYBZYVKFLw3R3E9K","level":2,"time":"2019-06-17T03:10:26+00:00","remoteAddr":"172.25.0.1","user":"--","app":"user_ldap","method":"GET","url":"\/ocs\/v2.php\/apps\/serverinfo\/api\/v1\/info?format=json","message":"Bind failed: 49: Invalid credentials","userAgent":"--","version":"15.0.5.3"}
{"reqId":"s0mNFYBZYVKFLw3R3E9K","level":2,"time":"2019-06-17T03:10:26+00:00","remoteAddr":"172.25.0.1","user":"--","app":"core","method":"GET","url":"\/ocs\/v2.php\/apps\/serverinfo\/api\/v1\/info?format=json","message":"Login failed: 'lfitt' (Remote IP: '172.25.0.1')","userAgent":"--","version":"15.0.5.3"}