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Sorry to hear you’re facing problems 
help.nextcloud.com is for home/non-enterprise users. If you’re running a business, paid support can be accessed via portal.nextcloud.com where we can ensure your business keeps running smoothly.
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, 12.0.2): 18.0.1
Operating system and version (eg, Ubuntu 17.04): Ubuntu 18.04.4
Apache or nginx version (eg, Apache 2.4.25): 2.4.27
PHP version (eg, 7.1): 7.4
The issue you are facing:
Log throwing errors on external storages:
[PHP] Error: Invalid characters passed for attempted conversion, these have been ignored at /var/www/nextcloud/apps/files_external/3rdparty/icewind/smb/src/Wrapped/Parser.php#149
PROPFIND /remote.php/dav/files/jk/Projekte
It throws same errors on all integrated external storages every 3-4 seconds.
Maybe the External Files App is not ready for PHP 7.4 or maybe a PHP 7.4 bug?
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
- Recently upgraded to NC 18.0.1 and PHP 7.4 but all went smoothly.
- Had a look at the log for other reason…
- Aside these errors all seems to be good.
The output of your Nextcloud log in Admin > Logging:
[PHP] Error: Invalid characters passed for attempted conversion, these have been ignored at /var/www/nextcloud/apps/files_external/3rdparty/icewind/smb/src/Wrapped/Parser.php#149
PROPFIND /remote.php/dav/files/jk/Projekte
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