I checked it. In the field "permission" of the request item, it says "Can't display permissions in this view".
It seems someone (accidently) deleted the system list "access requests". This list is re-created automatically when new request arrives, but, something is wrong.
It's not so easy to trouble shoot. In the end, when I deleted the "access requests" list and then sent out a new request, I got the error message from ULS log.
To fix it is easy. We need to delete the relevant web properties after deleting the "access requests" list. Or else, it caused error "key is already in the web property bag", which stopped the remaining steps. This list could be deleted from SharePoint designer.
Below is the PowerShell script to delete those two web properties.
$WebURL = "http://SiteUrl/sites/SiteName"
$key1 = "_VTI_ACCESSREQUESTSLISTID"
$key2 = "_VTI_PENDINGREQUESTSVIEWID"
$Web = Get-SPWeb -Identity $WebURL
$Web.AllowUnsafeUpdates = $true