ServiceUnavailable error on file

Hi, I am using NC 13.0.1.
One of my users noticed errors on some files, eg.

{"reqId":"eXOyrs2vzaT7nPNmJGON","level":4,"time":"2020-11-10T16:27:19+00:00","remoteAddr":"xxxx","user":"USERNAME","app":"webdav","method":"GET","url":"\/remote.php\/dav\/files\/USERNAME\/STRING%20Share\/STRING%202020\/MATERIALI\/DATI%20CATALOGO\/dati%20catalogo.docx","message":"Exception: {\"Exception\":\"Sabre\\\\DAV\\\\Exception\\\\ServiceUnavailable\",\"Message\":\"Could not open file\",\"Code\":0,\"Trace\":\"#0 \\\/var\\\/www\\\/nextcloud\\\/3rdparty\\\/sabre\\\/dav\\\/lib\\\/DAV\\\/CorePlugin.php(85): OCA\\\\DAV\\\\Connector\\\\Sabre\\\\File->get()\\n#1 [internal function]: Sabre\\\\DAV\\\\CorePlugin->httpGet(Object(Sabre\\\\HTTP\\\\Request), Object(Sabre\\\\HTTP\\\\Response))\\n#2 \\\/var\\\/www\\\/nextcloud\\\/3rdparty\\\/sabre\\\/event\\\/lib\\\/EventEmitterTrait.php(105): call_user_func_array(Array, Array)\\n#3 \\\/var\\\/www\\\/nextcloud\\\/3rdparty\\\/sabre\\\/dav\\\/lib\\\/DAV\\\/Server.php(479): Sabre\\\\Event\\\\EventEmitter->emit('method:GET', Array)\\n#4 \\\/var\\\/www\\\/nextcloud\\\/3rdparty\\\/sabre\\\/dav\\\/lib\\\/DAV\\\/Server.php(254): Sabre\\\\DAV\\\\Server->invokeMethod(Object(Sabre\\\\HTTP\\\\Request), Object(Sabre\\\\HTTP\\\\Response))\\n#5 \\\/var\\\/www\\\/nextcloud\\\/apps\\\/dav\\\/lib\\\/Server.php(283): Sabre\\\\DAV\\\\Server->exec()\\n#6 \\\/var\\\/www\\\/nextcloud\\\/apps\\\/dav\\\/appinfo\\\/v2\\\/remote.php(35): OCA\\\\DAV\\\\Server->exec()\\n#7 \\\/var\\\/www\\\/nextcloud\\\/remote.php(164): require_once('\\\/var\\\/www\\\/nextcl...')\\n#8 {main}\",\"File\":\"\\\/var\\\/www\\\/nextcloud\\\/apps\\\/dav\\\/lib\\\/Connector\\\/Sabre\\\/File.php\",\"Line\":337}","userAgent":"Mozilla\/5.0 (Windows) mirall\/3.0.1stable-Win64 (build 20200828) (Nextcloud)","version":"13.0.1.1"}

On server, on filesystem, such file is NOT present.
This user use 3.0.1stable-Win64 on Windows 10.
How can I troubleshoot this?
Thank you!

.