Cannot create ticket through email channel from a valid request type

Reproduce steps:

• Turn on Service Desk for HelpDesk.
• Turn on email channel for HelpDesk project with request type Bug - Detailed
• Send a normal email to that email channel.
• View the log to observe the result.

Error message received:

Your request could not be created. Please check the fields have been correctly filled in

Expected result:

• A new ticket should be successfully created.

Note that if I create new request through customer portal, everything still works normally.