The 404 error means that the event doesn't exist in the server. This can happen e.g. if you first delete the event from the server and then try to do the same from Rainlendar. Using an old offline copy can also cause the 404 error. In that case it is advised to reset the offline copy by disabling it first and enabling it after a refresh.
The .rainlendar2 folder contains all the settings and locally stored events. The problem with the Google Calendar is related how Rainlendar handles the response from the Googles server so the settings do not any impact on that. If you suspect that there is some incompatibility problem with the old settings you can create a portable setup
from the latest beta version.