Disconnection error question

I have 2 accounts attached to my battle.net but this problem only happens on my main account.

When I log out of every character I get the message

You have been disconnected. (WOW51900325)

I click on Okay and then log into the next character with no problems.

It’s not a major issue because I haven’t really been disconnected, but it is VERY annoying having to click on it before I can log into my next character. Anyone any ideas what’s causing it and how to fix it?