Hi,
In the Finder use the Go Menu whilst holding down the ALT key
Select the Library that appears now in the list.
Navigate to Preferences.
Drag com.apple.ichat.plist to the Trash and Restart Messages.
If that does not work repeat with com.apple.imagent.plist and Restart Messages.
Both of these are involved in the logged in state of the app and have details of the Accounts you have in Messages.
In doing this don't do them together.
Having said that neither will loose the account info in the app.
10:03 p.m. Tuesday; September 1, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad