Be aware of the behavior that an error occurs when the external user access from the invited email.
I am assuming that a system error (bugs) has been occurred when the external user access the link of sharing the file. Specifically, the access error constantly occurs for the first time the user accessed from the invited email, after that they are allowed to access without sign-in process.
For security protection, I hope this event will be fixed as soon as possible.
We fixed a bug here recently. Please let support know if you’re still seeing the issue. Thank you!