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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 ( 16.0.5):
Operating system and version (eg, ?):
Apache or nginx version (eg, ?):
PHP version (eg, ?):
The issue you are facing:
I can´t access the admin Web Panel inside my local network.
So nextcloudpi.local is working, while nextcloudpi.local:4443 is forbiden (403).
SSH is blocked as well, even so port 22 is open in my fritzbox.
It was possible to access the web-panel once, because I was able to set up DynDNS. But I can´t access it any longer.
Any Ideas?
Is this the first time you’ve seen this error? (Y/N):
Steps to replicate it:
The output of your Nextcloud log in Admin > Logging:
PASTE HERE
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