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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): 17.0.2
Operating system and version (eg, Ubuntu 17.04): Docker image
Apache or nginx version (eg, Apache 2.4.25): it’s in the docker container
PHP version (eg, 7.1): it’s in the docker container
The issue you are facing:
When I try to add a share link to a group folder via the API with basic auth, the request fails with error 404. A GET request works as well as sharing other normal folders of the user via the API. If I send a cookie of a browser session instead of the basic auth string along with the request, the stuff works.
Documentation: https://docs.nextcloud.com/server/17/developer_manual/client_apis/OCS/ocs-share-api.html
Is this the first time you’ve seen this error? (Y/N): Y
(when you mean compared to other versions; but had not tested it there)
Steps to replicate it:
Get the current shares:
curl -X GET -i -u "admin:mycoolpassword" https://mydomain.de/ocs/v2.php/apps/files_sharing/api/v1/shares -H "OCS-APIREQUEST:true"
-> Works like a charm and gives me a 200 OK with the currently active shares.
Add a share link for a group folder with basic auth:
curl -X POST -i -u "admin:mycoolpassword" https://mydomain.de/ocs/v2.php/apps/files_sharing/api/v1/shares -H "OCS-APIREQUEST:true" -d "shareType=3" -d "path=/testgroupfolder" -d "permissions=1"
–>
<?xml version="1.0"?>
<ocs>
<meta>
<status>failure</status>
<statuscode>404</statuscode>
<message>Can’t increase permissions of /admin/files/testgroupfolder</message>
</meta>
<data/>
</ocs>
Add a share link for a group folder with cookie:
curl -X POST -i https://mydomain.de/ocs/v2.php/apps/files_sharing/api/v1/shares -H "OCS-APIREQUEST:true" -H "Cookie: oc2ft4imfqls=3d068f3185055de71408a709cxxxxxxx; oc_sessionPassphrase=reDMqoWPswjr7uYPYOlyv2VILO05JRafzrFqTXJ%2BlOqFbZ4H%2BjBpllD9hTRfKR6KNSN4wEq%2FifxUV0wWAGG%2BqzwA5aqytJJ%2BYlsGwkwhjvg5o3oQO09PKbXXXXXXXXXX; nc_sameSiteCookielax=true; nc_sameSiteCookiestrict=true; nc_username=admin; nc_token=t4NqmrAoOHlQz%2BsrbKLtEsXXXXXXXXXX; nc_session_id=3d068f3185055de71408a7xxxxxxxxxx; __Host-nc_sameSiteCookielax=true; __Host-nc_sameSiteCookiestrict=true" -d "shareType=3" -d "path=/testgroupfolder" -d "permissions=1"
–>
<?xml version="1.0"?>
<ocs>
<meta>
<status>ok</status>
<statuscode>200</statuscode>
<message>OK</message>
</meta>
<data>
<id>3</id>
<share_type>3</share_type>
<uid_owner>admin</uid_owner>
<displayname_owner>admin</displayname_owner>
<permissions>1</permissions>
<stime>1577983705</stime>
<parent/>
<expiration/>
<token>jop4zy2iqmxxxxx</token>
<uid_file_owner>admin</uid_file_owner>
<note></note>
<label></label>
<displayname_file_owner>admin</displayname_file_owner>
<path>/testgroupfolder</path>
<item_type>folder</item_type>
<mimetype>httpd/unix-directory</mimetype>
<storage_id>local::/var/www/html/data/</storage_id>
<storage>1</storage>
<item_source>312</item_source>
<file_source>312</file_source>
<file_parent>6</file_parent>
<file_target>/testgroupfolder</file_target>
<share_with/>
<share_with_displayname/>
<password/>
<send_password_by_talk></send_password_by_talk>
<url>https://mydomain.de/s/jop4zy2iqmxxxxx</url>
<mail_send>1</mail_send>
<hide_download>0</hide_download>
</data>
</ocs>
The output of your Nextcloud log in Admin > Logging:
nothing related
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'htaccess.RewriteBase' => '/',
'memcache.local' => '\\OC\\Memcache\\APCu',
'apps_paths' =>
array (
0 =>
array (
'path' => '/var/www/html/apps',
'url' => '/apps',
'writable' => false,
),
1 =>
array (
'path' => '/var/www/html/custom_apps',
'url' => '/custom_apps',
'writable' => true,
),
),
'instanceid' => 'myid',
'passwordsalt' => 'supersalt',
'secret' => 'supersecret',
'trusted_domains' =>
array (
0 => '10.0.1.22',
1 => 'mydomain.de',
),
'trusted_proxies' =>
array (
0 => '10.0.1.22',
),
'datadirectory' => '/var/www/html/data',
'dbtype' => 'mysql',
'version' => '17.0.2.1',
'overwrite.cli.url' => 'http://10.0.1.22',
'overwriteprotocol' => 'https',
'dbname' => 'nextcloud',
'dbhost' => '10.0.1.10',
'dbport' => '',
'dbtableprefix' => 'oc_',
'mysql.utf8mb4' => true,
'dbuser' => 'nextcloud',
'dbpassword' => 'cooldbpass',
'installed' => true,
'mail_from_address' => 'noreply',
'mail_smtpmode' => 'smtp',
'mail_sendmailmode' => 'smtp',
'mail_domain' => 'myinternal.intranet',
'mail_smtphost' => '10.0.1.13',
'mail_smtpport' => '25',
'maintenance' => false,
'theme' => '',
'loglevel' => 2,
'skeletondirectory' => '',
);
-> But also tested on another instance with the same version.
The output of your Apache/nginx/system log in /var/log/____:
Nothing in the error logs. Access log:
"POST /ocs/v2.php/apps/files_sharing/api/v1/shares HTTP/1.0" 404 1553 "-" "curl/7.58.0"