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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:
Windows Client shuts under Win11 down w/o error message. It seems it crashes after the client has compared local and server data and should start the up- or download.
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): Version 3.4.1 (Windows)
Operating system and version (eg, Ubuntu 20.04): Windows 11 Pro
The issue you are facing:
Windows Client shuts under Win11 down w/o error message. It seems it crashes after the client has compared local and server data and should start the up- or download.
Is this the first time you’ve seen this error? (Y/N): N
Steps to replicate it:
- start client
- wait for comparison
- crash
The output of your Nextcloud log in Admin > Logging:
see DEBUG file uppon request