Recently I exported my Apple Calendar to the default .ics file format supported by Calendar.app. I then attempted to import that file into a new Google Calendar via Google's import UI (in calendar settings).

Unfortunately when I imported the calendar I received a cryptic error about Google Calendar being unavailable and to try again later. I had a feeling this wasn't actually the case and that the format of the file was likely the problem. Turns out I was right.

So, if you're exporting from Apple Calendar, you need to remove the attributes X-APPLE-STRUCTURED-LOCATION from the exported .ics file. Just open up the file in a text editor, find any instances of them, and remove the entire chunk for each attribute. For example, you might see events structured like this:

BEGIN:VEVENT
... (more stuff here) ...
X-APPLE-STRUCTURED-LOCATION;VALUE=URI;X-TITLE="1447 Ocean Avenue, Santa 
 Monica, CA 90401 US":
... (more stuff here) ...
END:VEVENT

Remove the entire chunk so it appears like this:

BEGIN:VEVENT
... (more stuff here) ...
END:VEVENT

You'll need to remove each instance of this attribute in the .ics file - there may be many of them.

Once you've removed them all, try and import via Google Calendar again.