If what you are saying is accurate, you are in the wrong folder - it should be C:\Users\XXX\.node-red
@Steve-Mcl thank you. I tried this, but with no positiv result.
I have even reinstalled windows once. I cannot install the node-red-contrib-ui-led library. when I install this is in the log file, it is successfully installed but it does not appear in the left bar to select this. Also, the editor says that I can still install this library, even though I have already done so.
Show us the output when you install it using the command line, including the line with the command so we can see the directory. Paste the log here using the </> button to paste it in.
Also then start node red in a terminal and post the startup log here.
CUsersXXX.node-rednpm install node-red-contrib-ui-led
+ node-red-contrib-ui-led@0.3.3
updated 1 package and audited 81 packages in 1.002s
found 0 vulnerabilities
CUsersXXX.node-rednode-red
18 Aug 021703 - [info]
Welcome to Node-RED
===================
18 Aug 021703 - [info] Node-RED version v1.1.3
18 Aug 021703 - [info] Node.js version v14.8.0
18 Aug 021703 - [info] Windows_NT 10.0.17763 x64 LE
18 Aug 021704 - [info] Loading palette nodes
18 Aug 021705 - [info] Dashboard version 2.23.2 started at ui
18 Aug 021705 - [info] Settings file UsersXXX.node-redsettings.js
18 Aug 021705 - [info] Context store 'default' [module=memory]
18 Aug 021705 - [info] User directory UsersXXX.node-red
18 Aug 021705 - [warn] Projects disabled editorTheme.projects.enabled=false
18 Aug 021705 - [info] Flows file UsersXXX.node-redflows_DESKTOP-8ICB8KS.json
18 Aug 021705 - [info] Server now running at http127.0.0.11880
18 Aug 021705 - [warn]
---------------------------------------------------------------------
Your flow credentials file is encrypted using a system-generated key.
If the system-generated key is lost for any reason, your credentials
file will not be recoverable, you will have to delete it and re-enter
your credentials.
You should set your own key using the 'credentialSecret' option in
your settings file. Node-RED will then re-encrypt your credentials
file using your chosen key the next time you deploy a change.
---------------------------------------------------------------------
18 Aug 021705 - [info] Starting flows
18 Aug 021705 - [info] Started flows
What browser are you using to open the node-red editor?
Can you look in the console for errors?
oh WOW! I have now quickly installed the edge browser and it works. I used Internet Explorer before. You're a genius. 
Yes, I suspected you might have be using Internet Explorer (or old edge).
Might be worth taking screen shots of the errors in the offending browser and raising an issue on the github repo for the DEV to understand there is an issue?
EDIT...
Curious as to why you were using IE. Is this a corporate environment?
Internet Explorer is an absolute dog & should be banished FOREVER 
IE11 was release in 2013! Web services have gradually ceased supporting it for several years now and it will finally be deprecated by Microsoft early next year.
Time to let go.
yes I write my master thesis in a company and use, among other things, Node red for an IoT project.
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