Yes, the other URL works also good when sent via the general HTTP module.
This should conclude that something is wrong on the Make.com implementation of Grist module. I will refer them to this thread. Thanks a lot for pushing it and resolving it 
Thanks for testing everything @Tomas_Westerholm!
Hi @paul-grist, Make replied to us. Have a look and let me know how to respond to them.
Grist API response for Get a document as Excel file_raw_anon.txt (28.5 KB)
Hi Tomas,
Thank you for choosing Make & reaching out to us. My name is Debbie from Customer Care team.
As I check from scenario debugger & based on Grist API docs, it looks like this is the error from Grist API since Make sent the request correctly, however the response doesn’t look like string data.
I have downloaded the raw response & request from Make, you may share this raw file (attached) to Grist Support team to have their thought & solution related to this issue.
Apologies for any inconvenience this may cause.
Let me know should you have any further questions related to this issue. If you need help with something else, please submit a new ticket and we will get back to you promptly.
Kind regards,
–
Debbie K
Your Make Team
@Tomas_Westerholm I don’t know how to help you. The response is an Excel file, which is a binary format, it is not string data. If it is being handled as string data, it would be very easy to corrupt it.
Is it possible there is another corner of make.com that specializes in binary files, and that the corner you are using has some assumption that it is dealing with text?
@paul-grist yes, if I encode the result into base64 binary, then it will actually form an Excel file which can be opened - but, the content is not correct. The file size is only 1kb. I will get back to Make.com for response.
@paul-grist Make.com confirmed the issue is on their code. They will fix it.
Hi Tomas,
Thanks for reaching out to Grist for possible solution & further investigation.
I have gone back & made a test myself from my own database & found that this is a bug issue from our module: the domain from the URL to make a GET request to Grist was duplicated. Therefore, it is unable to retrieve the correct data type for attachment. I am reporting the issue to our Dev team for the fix.
For the time being, would you mind trying to use our HTTP/ Make a request module in order to perform this action instead of using Grist/ Get document as Excel file module while we are fixing the issue. The request should be as follows:
URL: https://docs.getgrist.com/api/docs/{docId}/download/xlsx
Method: GET
The output from this module should be binary data type & you should be able to map over to your Microsoft 365 Email module as correct attachment type.
Kindly set up the module as above & run the test to see how it goes.
Once again, apologies for any inconvenience this may cause & we will give you an update once the current module is fixed.
Let me know should you have any further questions related to this issue. If you need help with something else, please submit a new ticket and we will get back to you promptly.
Kind regards,
–
Debbie K
Your Make Team