Update #2: This same problem was causing difficulties for some users to write comments on Flipnotes from personal computers, the Nintendo DSi Browser and mobile devices. The problem with comments has now been fixed as of 11:00 (JST) on January 21, 2013.
From approximately 7:15 am (JST) on January 19, 2013 we have been receiving reports of some Flipnote Hatena users having trouble connecting to the service.
"This user name cannot be used with Flipnote Hatena. Change your user name in the System Settings and try again."
We are aware of this problem and are currently trying to fix it.
We will do our best to fix this problem as soon as we can. Please accept our apologies for any inconvenience