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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:
"reqId":"YQeTQJNN6@FYnxlJ6iJ@0gAAABs","level":3,"time":"2021-08-02T06:40:01+00:00","remoteAddr":"10.3.6.4","user":"362453FB-4ABB-44F9-BC3D-DD54FF5F2AC0","app":"no app in context","method":"GET","url":"/index.php/apps/files/ajax/download.php?dir=%2F_00_Platforms%2F_PL_Semed%20-%20PLT%2F_SEMED_Report%20e%20CRM&files=SEMED%20-%20Analisi%20di%20Benchmark%20Finale_GD.pptx&downloadStartSecret=xoiv8zhh6t9","message":{"Exception":"OCP\\Encryption\\Exceptions\\GenericEncryptionException","Message":"Bad Signature","Code":0,"Trace":[{"file":"/logs/www/html/nextcloud/apps/encryption/lib/Crypto/Crypt.php","line":480,"function":"checkSignature","class":"OCA\\Encryption\\Crypto\\Crypt","type":"->","args":["*** sensitive parameters replaced ***"]},{"file":"/logs/www/html/nextcloud/apps/encryption/lib/Crypto/Encryption.php","line":376,"function":"symmetricDecryptFileContent","class":"OCA\\Encryption\\Crypto\\Crypt","type":"->","args":["*** sensitive parameter replaced ***",null,"AES-256-CTR","*** sensitive parameter replaced ***","*** sensitive parameter replaced ***"]},{"file":"/logs/www/html/nextcloud/lib/private/Files/Stream/Encryption.php","line":520,"function":"decrypt","class":"OCA\\Encryption\\Crypto\\Encryption","type":"->","args":["*** sensitive parameters replaced ***"]},{"file":"/logs/www/html/nextcloud/lib/private/Files/Stream/Encryption.php","line":318,"function":"readCache","class":"OC\\Files\\Stream\\Encryption","type":"->","args":[]},{"function":"stream_read","class":"OC\\Files\\Stream\\Encryption","type":"->","args":[8192]},{"file":"/logs/www/html/nextcloud/apps/files_external/3rdparty/icewind/streams/src/Wrapper.php","line":55,"function":"fread","args":[null,8192]},{"file":"/logs/www/html/nextcloud/apps/files_external/3rdparty/icewind/streams/src/CallbackWrapper.php","line":96,"function":"stream_read","class":"Icewind\\Streams\\Wrapper","type":"->","args":[8192]},{"function":"stream_read","class":"Icewind\\Streams\\CallbackWrapper","type":"->","args":[8192]},{"file":"/logs/www/html/nextcloud/lib/private/Files/View.php","line":432,"function":"fread","args":[null,524288]},{"file":"/logs/www/html/nextcloud/lib/private/legacy/OC_Files.php","line":353,"function":"readfile","class":"OC\\Files\\View","type":"->","args":["/_00_Platforms/_PL_Semed - PLT/_SEMED_Report e CRM/SEMED - Analisi di Benchmark Finale_GD.pptx"]},{"file":"/logs/www/html/nextcloud/lib/private/legacy/OC_Files.php","line":123,"function":"getSingleFile","class":"OC_Files","type":"::","args":[{"__class__":"OC\\Files\\View"},"/_00_Platforms/_PL_Semed - PLT/_SEMED_Report e CRM","SEMED - Analisi di Benchmark Finale_GD.pptx",{"head":false}]},{"file":"/logs/www/html/nextcloud/apps/files/ajax/download.php","line":64,"function":"get","class":"OC_Files","type":"::","args":["/_00_Platforms/******.pptx",{"head":false}]},{"file":"/logs/www/html/nextcloud/lib/private/Route/Route.php","line":156,"args":["/logs/www/html/nextcloud/apps/files/ajax/download.php"],"function":"require_once"},{"function":"OC\\Route\\{closure}","class":"OC\\Route\\Route","type":"->","args":["*** sensitive parameters replaced ***"]},{"file":"/logs/www/html/nextcloud/lib/private/Route/Router.php","line":310,"function":"call_user_func","args":[{"__class__":"Closure"},{"_route":"files_ajax_download"}]},{"file":"/logs/www/html/nextcloud/lib/base.php","line":993,"function":"match","class":"OC\\Route\\Router","type":"->","args":["/apps/files/ajax/download.php"]},{"file":"/logs/www/html/nextcloud/index.php","line":37,"function":"handleRequest","class":"OC","type":"::","args":[]}],"File":"/logs/www/html/nextcloud/apps/encryption/lib/Crypto/Crypt.php","Line":505,"Hint":"Bad Signature","CustomMessage":"--"},"userAgent":"Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:90.0) Gecko/20100101 Firefox/90.0","version":"21.0.3.1"}
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): 21.0.3
Operating system and version (eg, Ubuntu 20.04): CentOS 7
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.6
PHP version (eg, 7.4): 7.4.14
The issue you are facing:
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
The output of your Nextcloud log in Admin > Logging:
OCP\Encryption\Exceptions\GenericEncryptionException: Bad Signature
/logs/www/html/nextcloud/apps/encryption/lib/Crypto/Crypt.php - line 480:
OCA\Encryption\Crypto\Crypt->checkSignature("*** sensiti ... *")
/logs/www/html/nextcloud/apps/encryption/lib/Crypto/Encryption.php - line 376:
OCA\Encryption\Crypto\Crypt->symmetricDecryptFileContent("*** sensiti ... *", null, "AES-256-CTR", "*** sensiti ... *", "*** sensiti ... *")
/logs/www/html/nextcloud/lib/private/Files/Stream/Encryption.php - line 520:
OCA\Encryption\Crypto\Encryption->decrypt("*** sensiti ... *")
/logs/www/html/nextcloud/lib/private/Files/Stream/Encryption.php - line 318:
OC\Files\Stream\Encryption->readCache()
<<closure>>
OC\Files\Stream\Encryption->stream_read(8192)
/logs/www/html/nextcloud/apps/files_external/3rdparty/icewind/streams/src/Wrapper.php - line 55:
fread(null, 8192)
/logs/www/html/nextcloud/apps/files_external/3rdparty/icewind/streams/src/CallbackWrapper.php - line 96:
Icewind\Streams\Wrapper->stream_read(8192)
<<closure>>
Icewind\Streams\CallbackWrapper->stream_read(8192)
/logs/www/html/nextcloud/3rdparty/sabre/http/lib/Sapi.php - line 112:
stream_copy_to_stream(null, null, 243328)
/logs/www/html/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php - line 490:
Sabre\HTTP\Sapi::sendResponse(Sabre\HTTP\Response {})
/logs/www/html/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php - line 253:
Sabre\DAV\Server->invokeMethod(Sabre\HTTP\Request {}, Sabre\HTTP\Response {})
/logs/www/html/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php - line 321:
Sabre\DAV\Server->start()
/logs/www/html/nextcloud/apps/dav/appinfo/v1/webdav.php - line 84:
Sabre\DAV\Server->exec()
/logs/www/html/nextcloud/remote.php - line 167:
require_once("/logs/www/h ... p")```
The output of your config.php file in `/path/to/nextcloud` (make sure you remove any identifiable information!):
PASTE HERE
The output of your Apache/nginx/system log in `/var/log/____`:
PASTE HERE