[horde] Horde imp logging out

[Laura McCord]

We have had a large number of users saying that they are being logged out of the webmail system multiple times. They have been receiving the following message:

"Your Internet Address has changed since the beginning of your Mail
session. To protect your security, you must login again.
Username"

Can someone help me identify why this is happening? Is this a browser type and version issue? I am a loss at what is happening. 

We are using Horde 3.1.3

Thanks.