TL;DR version: If you encounter problems with GoToWebinar calendar entries on a Mac remove the ORGANIZER line from the file and try again. The full explanation follows.

When you sign up for a Brent Ozar or PASS Virtual Chapter webinar you'll receive an email confirming it:

The Add to Calendar link is a URL to an .ics file at GoToWebinar which will save to ~/Downloads.

Then it runs Calendar and prompts asking if you want to add it.

And then it eats dirt. This is current up to Mac OS X Mavericks 10.9.1.

After some experimentation with Apple's not-so-great button descriptions here is a rough translation:

Ignore Prompt me with the same error in a few minutes time.
Try Again Prompt me with the same error immediately.
Revert to Server Delete any calendar entries with this error right now.

If you search Google for "403 CalDAVWriteEntityQueueableOperation" you'll find a lot of similar complaints with calendars over the years and just as many attempted fixes. This is an example of a file it is trying to process:

...
BEGIN:VEVENT
DTSTAMP:20140116T113610Z
DTSTART;TZID=America/Chicago:20140128T113000
DTEND;TZID=America/Chicago:20140128T120000
SUMMARY:GoToWebinar - Reporting in Production with SQL Server
LOCATION:GoToWebinar - See conference call information below
UID:[email protected]
ORGANIZER:[email protected]
...

Some of the attempted fixes involved altering the UID value, removing the UID value, or removing the UID line entirely. This worked for 2/5 of the above calendars.

What worked for all of them was to remove the ORGANIZER line before opening it in Calendar. If you search Google for "403 CalDAVWriteEntityQueueableOperation ORGANIZER" it appears there may be an expectation from Apple that this field include more than just an email address.

I haven't dug any further into that. But I do often download and use calendars and so wrote up a quick bash script to remove that line for me:

#!/bin/bash
shopt -s nullglob       # If no files exist, the for loop exits

for f in ~/Downloads/*.ics
do
        echo "Processing $f"
        grep -vw ORGANIZER "$f" > "${f}_temp"
        mv "${f}_temp" "$f"
        open "$f"
done

Evidence it worked:

Now I just need to stay up late enough to actually watch the webinars.