Hi @Nico Van Gils
Thanks for your patience.
Based on my research, please ensure your link is under Secret address in iCal format, as shown in the image below:
I found that a link that is not this will cause the error you described, please re-copy your link and add your calendar by going to your Outlook calendar>click Add calendar>From Internet and the paste the link.
If the addition still fails, it is recommended that you disable Shared Calendar Improvements (File > Account Settings > Account Settings>select your account, Change>More Settings > Advanced >uncheck Turn on shared calendar improvements) and restart Outlook to add again to see if it works.
In addition, you may try to add your calendar by exporting and importing. More information: See your Google Calendar in Outlook - Microsoft Support
Hope it helps.
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