Since 17th of November the Google API used by the tool seems to be deprecated and therefore sync fails with Google - see e.g. https://wordpress.org/support/topic/403-forbidden-on-cal-url-deprecated-api
See this comment:
https://sourceforge.net/p/lntogoogle/discussion/1072332/thread/506390fc/#afca
switched to Google API v3
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See this comment:
https://sourceforge.net/p/lntogoogle/discussion/1072332/thread/506390fc/#afca
switched to Google API v3