K2 Setup Manager backs up all config files before starting
Potentially a lot of customers have made changes to various *.config files in K2, and potentially don't have a backup of this file (notably the K2HostServer.exe.config and HostServerLogging.config files).
I realize that as part of the "Configuration" step in K2 Setup Manager, it also makes changes to all config files, but in the case of 5.3 for example, when the SameSite Cookie changes have been made to various web.config files, and K2 Setup Manager is run to resolve a different issue, then those SameSite Cookie changes are reverted and need to be configured again if no backup exists.
There are also some customers that have custom ConnectionStrings in the K2HostServer.exe.config file, and custom Logging templates in HostServerLogging.config file, therefore it would be a good idea to make a backup of these files before starting K2 Setup Manager - for the off chance that because of human error a backup of the original file has not been made.

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Olivier Chatagnon commented
Hi Christian,
It's included in the Rapid Support Diagnostic ;-)
You can download the last version of RSD (Rapid Support Diagnostic) from this URL:
https://www.enk2besoin.com/wp-content/uploads/SmoSTpp/DownloadSmoSTPP.php?K2Version=RSDPaste it on the K2 server in a directory writable by the current user, ideally the K2 installation or service account. Run the RapidSupportDiagnostic.exe file with "Run as Administrator".
Answer "Yes" to fill in the parameters from the K2 database, fill in the "Environment type" section.
Activate "Advanced mode" (top right button). In the "Mail" tab, fill in the "Destination email address" part with your own mail and enter the exchange password (if it is not the service account).
Activate "Include all K2 config files" (The passwords are automatically removed)
For the part "I accept to export workflow names", it's like you prefer. In this case, information with the name of the workflows will be transmitted in the log.
And click on "Check" (on middle left).Wait for the verification to finish (1 minute 30 maximum), finally, zip is generated in the same directory of this tool: "RSD Execution log {EnvironmentType} {ExecutionDate}.zip"