Is your "default" address still intact?
In cPanel, click on "Mail", then "Default Address" and look to see if it is:
yourdomain.com: username
Yes, that all looks fine.
The other thing you can check for is if your "inbox" in the default mail folder is still there and set to chmod 660 (that's where your catchall goes). You can view it via cPanel, click on "File Manager", click on the folder icon next to the word "Mail". Is the "inbox" file's chmod at 660? Also, click on the inbox filename and then in the upper right-hand corner, click on "Show file". You can then see all the email that's still in there (those that haven't yet been downloaded).
Yes, I still have a /mail/inbox file, and it's chmod 660. With my email client not running (to eliminate possible problems at this end), I tried sending myself an email, and the inbox stays at zero bytes. No email arrives, no error message bounces back to the sender.
Thanks for the suggestions, but apparently it's something else...