Issue
When running the K2 Connection String Editor, the following error is thrown:
System.Configuration.ConfigurationErrorsException: Failed to decrypt using provider 'K2ConfigurationKey'. Error message from the provider: The RSA key container could not be opened. (E:Program FilesK2Host ServerBinK2HostServer.exe.config line 497) ---> System.Configuration.ConfigurationErrorsException: The RSA key container could not be opened.
Error Code

Resolution
Right click the ConnectionStringEditor.exe and select Run as Administrator and this will resolve the issue.
Additional Information
https://community.nintex.com/t5/K2-Archived-Articles/Troubleshooting-common-email-errors/ta-p/177775