"Your session is invalid or expired" error via DSL, works fine via 3G, EDGE, etc

[TheWalrus]

Hi all

I hope I'm putting this in the right place - I apologise if not.

I'm a Roundcube end-user, and am having a seriously frustrating time of things at the moment. My ISP uses Roundcube as their webmail handler, but since January of this year I have been unable to log in on any of the computers in my house using a DSL connection.

There are 4 PCs in total, and when I attempt to log in with any of them I receive a "Your session is invalid or expired" error. This occurs irrespective of OS (Win 7, Win 8 and Ubuntu) or browser (IE, Firefox, Chrome). The outcome is unaffected whether the connection is made through ethernet or wifi to the DSL router.

I can log in with no trouble on either of 2 smartphones. It also works fine if I use either smartphone as a modem for any of the 4 PCs in question. So the problem correlates with the DSL connection.

My ISP has been utterly useless in assisting, and are unable to help beyond the ususal "reboot the router and try again" type of advice.

Could anybody here shed some light on the matter?

Many thanks 

[SKaero]

I don't really see how the access method could change how RoundCube acts, any access method would still be using TCP/IP to connect to RoundCube. It clearly sounds like a browser problem but the fact that it works on other connections (3G & EDGE) with the same computers should meen its not that. Then the only differences then should be the IP address which would point to some sort of firewall blocking, but you can access the RoundCube login so it shouldn't be that. I've seen the same error before if with certain web servers but again that shouldn't be the case since it works in other places. My only guess is you DSL router it doing some sort of proxying, filtering or possibly connecting to a different server that is broken.