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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, 20.0.5): 27.1.0
Operating system and version (eg, Ubuntu 20.04): Ubuntu 20.04
Apache or nginx version (eg, Apache 2.4.25): Apache/2.4.41
PHP version (eg, 7.4): PHP 8.1.21
The issue you are facing:
I recently upgrades to 27.1.0, and just before this, I also applied the recent update to the Group Folders app. Now (and I don’t know if it’s connected to the upgrade) one of my users has lost access to a group folder. I haven’t changed anything about his permissions or groups. I’m also using Files Acces Controls on this folder, I haven’t changed anything about that either.
Is this the first time you’ve seen this error? (Y/N): Y
An example error in the log is shown below:
Sep 20 10:27:13 ed-mh-x86001 Nextcloud[3903645]: {"reqId":"bZ5E05uSR2l8kXShqjkR","level":3,"time":"2023-09-20T09:27:13+00:00","remoteAddr":"<redacted>","user":"<redacted>","app":"webdav","method":"PROPFIND","url":"/remote.php/dav/files/<redacted>/REOS-SHARED","message":"{\"Exception\":\"OCA\\\\DAV\\\\Connector\\\\Sabre\\\\Exception\\\\Forbidden\",\"Message\":\"No read permissions. This might be caused by files_accesscontrol, check your configured rules\",\"Code\":0,\"Trace\":[{\"file\":\"/var/www/nextcloud/apps/dav/lib/Connector/Sabre/FilesPlugin.php\",\"line\":475,\"function\":\"getChildren\",\"class\":\"OCA\\\\DAV\\\\Connector\\\\Sabre\\\\Directory\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/event/lib/WildcardEmitterTrait.php\",\"line\":89,\"function\":\"handleGetProperties\",\"class\":\"OCA\\\\DAV\\\\Connector\\\\Sabre\\\\FilesPlugin\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php\",\"line\":1052,\"function\":\"emit\",\"class\":\"Sabre\\\\DAV\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php\",\"line\":984,\"function\":\"getPropertiesByNode\",\"class\":\"Sabre\\\\DAV\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php\",\"line\":1662,\"function\":\"getPropertiesIteratorForPath\",\"class\":\"Sabre\\\\DAV\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php\",\"line\":1647,\"function\":\"writeMultiStatus\",\"class\":\"Sabre\\\\DAV\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/CorePlugin.php\",\"line\":346,\"function\":\"generateMultiStatus\",\"class\":\"Sabre\\\\DAV\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/event/lib/WildcardEmitterTrait.php\",\"line\":89,\"function\":\"httpPropFind\",\"class\":\"Sabre\\\\DAV\\\\CorePlugin\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php\",\"line\":472,\"function\":\"emit\",\"class\":\"Sabre\\\\DAV\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php\",\"line\":253,\"function\":\"invokeMethod\",\"class\":\"Sabre\\\\DAV\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php\",\"line\":321,\"function\":\"start\",\"class\":\"Sabre\\\\DAV\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/apps/dav/lib/Server.php\",\"line\":365,\"function\":\"exec\",\"class\":\"Sabre\\\\DAV\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/apps/dav/appinfo/v2/remote.php\",\"line\":35,\"function\":\"exec\",\"class\":\"OCA\\\\DAV\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/remote.php\",\"line\":172,\"args\":[\"/var/www/nextcloud/apps/dav/appinfo/v2/remote.php\"],\"function\":\"require_once\"}],\"File\":\"/var/www/nextcloud/apps/dav/lib/Connector/Sabre/Directory.php\",\"Line\":257,\"message\":\"No read permissions. This might be caused by files_accesscontrol, check your configured rules\",\"exception\":{},\"CustomMessage\":\"No read permissions. This might be caused by files_accesscontrol, check your configured rules\"}","userAgent":"Mozilla/5.0 (Windows) mirall/3.9.2stable-Win64 (build 20230808) (Nextcloud, windows-10.0.22621 ClientArchitecture: x86_64 OsArchitecture: x86_64)","version":"27.1.0.7"}
