I investigated the errors reported on this ticket and pretty much all of them were already fixed when the new help center experience was released.
I've also checked https://getsupport.atlassian.com/browse/PSCLOUD-47123 and noticed that the 403 error happens when calling `/servicedesk/customershim/secure/attachment`
The only way that this URL is can be called is through the links in the email notification.
I managed to reproduce the error by logging into the help center with a user that has access to the help center but not access to the issue.

There are a two possibilities that can make this happen:
1 - The user that follows the link is logged in with a different account from the one that created the task.
2 - The user that follows the link used to be one of the participants in the request but was removed
So in this case, I don't believe this is actually an error but a real lack of permissions.
cc d4f60db6c681
Closing this ticket since the last investigations proved that the errors linked to this ticket were either fixed in the new help center or are an actual lack of permissions.