Nextcloud-Android app: sharing external folder between internal users

Hi,

I am new to the forum and just wonder why I can easily share an external folder. Sharing icons and menu entries are visible and accessible.

On the server I am running NC 18.0.7.

Working sharing via the web-gui in Firefox:

nnn.nnn.nnn.nnn - - [31/Jul/2020:12:31:38 +0200] "GET /ocs/v2.php/apps/notifications/api/v2/notifications HTTP/2.0" 200 74 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:32:08 +0200] "GET /ocs/v2.php/apps/notifications/api/v2/notifications HTTP/2.0" 200 74 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:32:40 +0200] "GET /ocs/v2.php/apps/notifications/api/v2/notifications HTTP/2.0" 200 74 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:32:40 +0200] "GET /index.php/apps/files/ajax/getstoragestats.php?dir=%2FXXXXXX HTTP/2.0" 200 223 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:04 +0200] "PROPFIND /remote.php/dav/systemtags-relations/files/120826 HTTP/2.0" 207 428 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:05 +0200] "PROPFIND /remote.php/dav/systemtags-relations/files/120826 HTTP/2.0" 207 428 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:06 +0200] "GET /ocs/v2.php/collaboration/resources/file/120826?format=json HTTP/2.0" 200 74 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:06 +0200] "REPORT /remote.php/dav/comments/files/120826/ HTTP/2.0" 207 156 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:06 +0200] "GET /ocs/v2.php/apps/activity/api/v2/activity/filter?format=json&object_type=files&object_id=120826 HTTP/2.0" 304 0 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:06 +0200] "GET /ocs/v2.php/apps/files_sharing/api/v1/shares?format=json&path=%2FXXXXXX%2FYYYYYYYYY&reshares=true HTTP/2.0" 200 74 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:06 +0200] "GET /ocs/v2.php/apps/files_sharing/api/v1/shares?format=json&path=%2FXXXXXX%2FYYYYYYYYY&shared_with_me=true HTTP/2.0" 200 74 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:06 +0200] "GET /ocs/v2.php/apps/notifications/api/v2/notifications HTTP/2.0" 200 74 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:06 +0200] "GET /ocs/v1.php/apps/files_sharing/api/v1/sharees_recommended?format=json&itemType=dir HTTP/2.0" 200 324 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:11 +0200] "POST /ocs/v2.php/apps/files_sharing/api/v1/shares HTTP/2.0" 200 685 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:11 +0200] "GET /ocs/v1.php/apps/files_sharing/api/v1/sharees_recommended?format=json&itemType=dir HTTP/2.0" 200 324 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:05 +0200] "PROPFIND /remote.php/dav/systemtags-relations/files/120826 HTTP/2.0" 207 428 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:06 +0200] "GET /ocs/v2.php/collaboration/resources/file/120826?format=json HTTP/2.0" 200 74 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:06 +0200] "REPORT /remote.php/dav/comments/files/120826/ HTTP/2.0" 207 156 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:06 +0200] "GET /ocs/v2.php/apps/activity/api/v2/activity/filter?format=json&object_type=files&object_id=120826 HTTP/2.0" 304 0 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:06 +0200] "GET /ocs/v2.php/apps/files_sharing/api/v1/shares?format=json&path=%2FXXXXXX%2FYYYYYYYYY&reshares=true HTTP/2.0" 200 74 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:06 +0200] "GET /ocs/v2.php/apps/files_sharing/api/v1/shares?format=json&path=%2FXXXXXX%2FYYYYYYYYY&shared_with_me=true HTTP/2.0" 200 74 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:06 +0200] "GET /ocs/v2.php/apps/notifications/api/v2/notifications HTTP/2.0" 200 74 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:06 +0200] "GET /ocs/v1.php/apps/files_sharing/api/v1/sharees_recommended?format=json&itemType=dir HTTP/2.0" 200 324 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:11 +0200] "POST /ocs/v2.php/apps/files_sharing/api/v1/shares HTTP/2.0" 200 685 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:11 +0200] "GET /ocs/v1.php/apps/files_sharing/api/v1/sharees_recommended?format=json&itemType=dir HTTP/2.0" 200 324 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"
nnn.nnn.nnn.nnn - - [31/Jul/2020:12:35:35 +0200] "GET /ocs/v2.php/apps/notifications/api/v2/notifications HTTP/2.0" 200 74 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0"

and fails with 404 when I try sharing a folder via the Android-Nextcloud:

nnn.nnn.nnn.nnn - uuuuuuuu [31/Jul/2020:12:31:15 +0200] "GET /ocs/v1.php/cloud/user?format=json HTTP/1.1" 200 576 "-" "Mozilla/5.0 (Android) Nextcloud-android/3.12.1"
nnn.nnn.nnn.nnn - uuuuuuuu [31/Jul/2020:12:31:16 +0200] "PROPFIND /remote.php/webdav/XXXXXXX/ HTTP/1.1" 207 833 "-" "Mozilla/5.0 (Android) Nextcloud-android/3.12.1"
nnn.nnn.nnn.nnn - uuuuuuuu [31/Jul/2020:12:31:22 +0200] "PROPFIND /remote.php/webdav/XXXXXXX/ HTTP/1.1" 207 155554 "-" "Mozilla/5.0 (Android) Nextcloud-android/3.12.1"
nnn.nnn.nnn.nnn - uuuuuuuu [31/Jul/2020:12:31:24 +0200] "GET /ocs/v2.php/apps/files_sharing/api/v1/shares?path=%2FXXXXXXX%2F&reshares=true&subfiles=true HTTP/1.1" 200 2248 "-" "Mozilla/5.0 (Android) Nextcloud-android/3.12.1"
nnn.nnn.nnn.nnn - uuuuuuuu [31/Jul/2020:12:31:24 +0200] "GET /ocs/v2.php/apps/activity/api/v2/activity/filter?object_type=files&format=json&previews=true&sort=desc&object_id=120867 HTTP/2.0" 304 0 "-" "Mozilla/5.0 (Android) Nextcloud-android/3.12.1"
nnn.nnn.nnn.nnn - uuuuuuuu [31/Jul/2020:12:31:24 +0200] "PROPFIND /remote.php/dav/versions/uuuuuuuu/versions/120867 HTTP/1.1" 404 192 "-" "Mozilla/5.0 (Android) Nextcloud-android/3.12.1"
nnn.nnn.nnn.nnn - uuuuuuuu [31/Jul/2020:12:31:27 +0200] "GET /ocs/v2.php/cloud/capabilities?format=json HTTP/1.1" 200 3098 "-" "Mozilla/5.0 (Android) Nextcloud-android/3.12.1"
nnn.nnn.nnn.nnn - uuuuuuuu [31/Jul/2020:12:31:38 +0200] "GET /ocs/v2.php/apps/files_sharing/api/v1/sharees?format=json&itemType=file&search=a&page=1&perPage=50 HTTP/1.1" 200 11798 "-" "Mozilla/5.0 (Android) Nextcloud-android/3.12.1"
nnn.nnn.nnn.nnn - uuuuuuuu [31/Jul/2020:12:31:41 +0200] "GET /ocs/v2.php/apps/files_sharing/api/v1/sharees?format=json&itemType=file&search=assssssss&page=1&perPage=50 HTTP/1.1" 200 310 "-" "Mozilla/5.0 (Android) Nextcloud-android/3.12.1"
nnn.nnn.nnn.nnn - uuuuuuuu [31/Jul/2020:12:31:47 +0200] "GET /ocs/v2.php/apps/files_sharing/api/v1/sharees?format=json&itemType=file&search=assssssss&page=1&perPage=50 HTTP/1.1" 200 310 "-" "Mozilla/5.0 (Android) Nextcloud-android/3.12.1"
nnn.nnn.nnn.nnn - uuuuuuuu [31/Jul/2020:12:31:51 +0200] "GET /ocs/v2.php/apps/files_sharing/api/v1/sharees?format=json&itemType=file&search=ssssssss&page=1&perPage=50 HTTP/1.1" 200 439 "-" "Mozilla/5.0 (Android) Nextcloud-android/3.12.1"
nnn.nnn.nnn.nnn - uuuuuuuu [31/Jul/2020:12:31:53 +0200] "POST /ocs/v2.php/apps/files_sharing/api/v1/shares HTTP/1.1" 404 211 "-" "Mozilla/5.0 (Android) Nextcloud-android/3.12.1"

Due to the working webgui-access I would expect no problem with file permission or user user rights.

Any helpful hints would be greatly apreciated.