Hello dear community,
my problem is that i can't connect (LAN) to the interface with any browser.
With SSH (Putty) it still works.
I try to reset to default settings with "firstboot" but it doesn't work.
("/dev/mtdblock3 is mounted as / overlay, only erasing files")
I still can't connect to the interface with a browser.
What can i try know - PLEASE HELP ME 
It sounds like you installed a snapshot which doesn't come with a gui (only stable versions have gui), so ssh in and run -
opkg update
opkg install luci
Then you should have a gui.
Yes that may be possible.
Lede was the OS before Open-Wrt but instead to flash with "factory" (because it's new OS) i do it with "sysupgrade" because foolishly in my mind it was an update 
.....
Thx i'll try it now!
You used the right file, sysupgrade is for going from LEDE to Openwrt or Openwrt to Openwrt, factory is for going from stock firmware to Openwrt/LEDE.
Now i have a gui and can connect with a browser to the web interface but:
"No password set! There is no password set on this router. Please configure a root password to protect the web interface and enable SSH."
But i can't set a password when i click on "Go to password configuration".
Error message: "404 Not Found: Sorry, the object you requested was not found. Unable to dispatch: /cgi-bin/luci/admin/system/admin"
You can set a password by running the passwd command in SSH.
What do you have in SSH, when you give the command opkg list-installed | grep luci ?
Puhh thanks I thought I did everything wrong.......
I had even made a backup of LEDE because of the settings but I can probably no longer use it or?
Even if not synonymous, that's done quickly.
But what went wrong if I did not make the mistake?
Of course you can use it. The only thing that cannot be restored is the installed packages. For that you would need to have created already a list of them and restore them when your new installation is up.
When i want to set a password in SSH:
"passwd: can't update password file /etc/passwd"
Command opkg list-installed | grep luci said:
Thx 4 your answer!
I have already downloaded the upgrade.
The problem still is that i can't login to the Web-Interface.
Or is there another way to install the sysupgrade?
Are you sure you're using the stable 18.06.1 build that @lleachii suggested? That's the one that comes with an interface, you can find it here.
How can i install the sysupgrade when i can't login to the Web-Interface?
Is there another way to install the sysupgrade like SSH (Putty)?
I'm sure the script blockers on my browser do not cause any problems and have nothing to do with it.
Yes i think so "openwrt-18.06.1-ar71xx-generic-archer-c7-v2-squashfs-sysupgrade.bin" << i hope this is a stable build!?
The sysupgrade -n command will upgrade wihtout saving defaults on the command line. You will wget the file to /tmp directory and then run the command.
Yes, that's the right one.
The command sysupgrade -n open up the upgrade-options.
What i have to do next?
Sry i don't want to make a mistake now and want to fix the problem finally soon!
You did download a file to use the sysupgrade command on, correct?
The file where you confirmed it was the right one?
How can i install this file without a functional webinterface.
Or how i can do it with SSH < step by step PLZ .... I'm not very good at handling commands......