Unable to upload or create new directory in some folders

Nextcloud version: 19
Operating system and version: Ubuntu 20.04 on Google Cloud VM
Apache or nginx version: Apache 2.4.41
PHP version: 7.4.3

The issue you are facing:
I am able to create new folders and upload files via web browser and/or iOS client, except in one particular folder. The only thing special about that folder is that a share link has been created for it.

I have tried creating a folder in / and moving the newly created folder into the target folder, still couldn’t; it simply gave me a “Could not move” error.

When trying to upload or create folder within the target folder, it gave only “An unknown error occurred”.

Is this the first time you’ve seen this error?
Yes, I was able to upload and create folders before.

The output of your Nextcloud log in Admin > Logging:

None related; last log was a login failure (I typed the wrong pw), and the second one was 5 days ago.

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

<?php
$CONFIG = array (
  'passwordsalt' => ' ',
  'secret' => ' ',
  'trusted_domains' => 
  array (
    0 => 'localhost',
    1 => 'x.x.x.x',
    2 => 'cloud.x.com',
    3 => 'cloud.x.com',
  ),
  'datadirectory' => '/mnt/ncdata',
  'dbtype' => 'pgsql',
  'version' => '19.0.2.2',
  'overwrite.cli.url' => 'https://cloud.x.com/',
  'dbname' => 'nextcloud_db',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'ncadmin',
  'dbpassword' => ' ',
  'installed' => true,
  'instanceid' => ' ',
  'upgrade.disable-web' => 'true',
  'log_type' => 'file',
  'logfile' => '/var/log/nextcloud/nextcloud.log',
  'loglevel' => '2',
  'log.condition' => 
  array (
    'apps' => 
    array (
      0 => 'admin_audit',
    ),
  ),
  'mail_smtpmode' => 'smtp',
  'remember_login_cookie_lifetime' => '1800',
  'log_rotate_size' => '10485760',
  'trashbin_retention_obligation' => 'auto, 180',
  'versions_retention_obligation' => 'auto, 365',
  'simpleSignUpLink.shown' => false,
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'filelocking.enabled' => true,
  'memcache.distributed' => '\\OC\\Memcache\\Redis',
  'memcache.locking' => '\\OC\\Memcache\\Redis',
  'redis' => 
  array (
    'host' => '/var/run/redis/redis-server.sock',
    'port' => 0,
    'timeout' => 0.5,
    'dbindex' => 0,
    'password' => ' ',
  ),
  'logtimezone' => 'Asia/Hong_Kong',
  'htaccess.RewriteBase' => '/',
  'maintenance' => false,
  'mail_from_address' => 'x',
  'mail_sendmailmode' => 'smtp',
  'mail_domain' => 'gmail.com',
  'mail_smtphost' => 'smtp.gmail.com',
  'mail_smtpport' => '465',
  'mail_smtpsecure' => 'ssl',
  'mail_smtpauth' => 1,
  'mail_smtpname' => 'x@gmail.com',
  'mail_smtppassword' => ' ',
  'mail_smtpauthtype' => 'LOGIN',
);

The output of your Apache/nginx/system log in /var/log/____:

Again, none related. Was encountering the error on 10 Sep but only log on 10 Sep is about my wrong pw login.

Thanks!