Dear Peter,
We are glad to help you and need to fully confirm the issue in your environment for troubleshooting.
The Google Drive link isn't an Anyone link. When I enter it, I see the following message.
If it is convenient, could you upload some related screenshots here instead? You can click on Reply>Insert image to see whether the screenshots can be uploaded successfully.
About "The problem is that what ever I enter in the field 'User can request additional permissions from:' nothing is saved. If I leave the field empty nothing is saved. In short: what ever I do it looks as before when I open the file and it Permissions the next time. Always the same mailto: link and the strange > at the end.", some related screenshots are also appreciated.
If it is convenient, please open a new file>click on File>Account and upload a full screenshot of the Account page in Excel for our reference.
Note: Remove nay private information before uploading the screenshots.
Meanwhile, I remove the links to get rid of any confusion.
As the file is enabled with macro, is the original file owner in your company now? If yes, may I know whether the original file owner can open the file normally from his side? How do you inherit the file?
If he can still open the file, could you let her temporarily remove the macro in the file and then see whether you can open the file from your side?
Welcome to share any updates at your convenience.
Thank you for your time and effort.
Sincerely
Cliff | Microsoft Community Moderator