I recently changed my Minecraft username and now whenever I try to log into either DMU Public or DMU Beta, an error message saying "Failed to verify username!". What can I do to fix this problem?

Log Out of the Launcher then log back in

It worked, thank you for the help.

what do i have to do when it writes to me:           failed to login: null 

 Did you bought Minecraft or do you have it cracked?

Cracked

That is the cause you can't join DMU. We don't support cracked Minecraft because that is against the EULA. If you wanna play on DMU you need a legal Minecraft account.

I also tried legal minecraft and it also wrote to me