'Remember me' not saved

I've recently installed IF 4.1.3 (http://www.autokosten.info/InstantForum413) and everyone is experiencing the following issue:

When logging in, you can check the Remember me option. If you do, and after being logged in you close the browser, restart it and return to the forum, you're still logged in (so far so good). However, after 1-12 hours (unfortunately this varies wildly per user) he/she has to login again. If you check the forum once a day, you'll have to login every day...

There already are a lot of posts about this on this forum, and I've tried them all (I think :) ):

-Adding a timeout value to the authentication part in web.config
-Installed IF in the root of the website, instead of the folder /InstantForum413/ it is in now
-Changed the path attribute of the authentication part in web.config to path="/InstantForum413/"
-Tried running IF under .NET Framework 1.1 and 2.0
-I've deleted all cookies on the computer and tried again
-I've tried it on several different computers

Also something strange, which may be related: if you login, and then press logout, you get the message that your cookie will be deleted. Press OK to confirm.. the page reloads, and you find yourself still logged in.

The current web.config authentication part :

<authentication mode="Forms">

<forms name="InstantASP" loginUrl="Logon.aspx" protection="All" slidingExpiration="true" path="/InstantForum413/"/>

</authentication>

I've ran out of ideas...

I've contacted support, and Carlos has confirmed this problem, but unfortunately he was unable to determine what the problem is at this time.

Is there anyone who has a suggestion I can try? (Feel free to try it yourself on the forum...)

Best regards,
Leon