Please try this: first, run Line 6 Monkey. If you've got 1.50 installed, let's roll you back to 1.43.

Monkey 1.43 Installer Windows:

http://line6.com/getrelease?rid=4223

Monkey 1.43 Installer Mac:

http://line6.com/getrelease?rid=4222

What we're doing here is to revert your Monkey software to the previous version. If you see a message appear saying there's a newer version of Monkey (would you like to update?) please CANCEL/disregard.

Once you have Monkey 1.43 installed, connect your HD400 via USB and select the Flash Memory component in the Updates list and "Reinstall Latest".

Please don't forget to do a Reset after the Monkey re-flash. What I mean by this is the HD400 reset, outlined here: http://line6.com/support/docs/DOC-2043

Re-test your device. That symptom should not re-occur.