Players may have to LOGOUT and LOGIN due to the user interface functions failing.
There are a number of different signs your server connection needs to be refreshed, as follows:

You see 75 LICENSES in the house icon (top center, second from left); if it is 75 logout and login again. Only a player who has no buildings other than their Mayor's House will be likely to actually have 75 licenses, or a player who has purchased enough licenses to hit 75 again at some point.
You hear only SILENCE when you should be hearing buff productions chiming.
FEWER THAN FOUR TABS IN CHAT. New players should always have FOUR TABS, players with a Branch Office for trading or who have joined a guild should always have a minimum of Five tabs, and players with both should have SIX.
Chat or Trade appears HUNG or FROZEN.
BUFFING FAILS, lags, or takes too long.
MAP SCROLLS CONTINUOUSLY TO EDGE and you can't stop the scrolling.
EMPTY MAIL WINDOW (unless you are in the habit of deleting all your mails).

That's all we can think of fer now, anyone else got a common error that is easy to notice and requires relogging?

Enjoy!

PS to UBI staff, we would set up a filter for logout-login events that are consecutive within 3-5 minutes of each other if you need data on how many users are experiencing these issues.