Active Sync error 0x85010018 on WM6.1 devices (time out)

[chh]

Hi,
we have a problem syncing mails to 2 of our WM6.1 devices. We get the error code 0x85010018. According to a MS page it means that there is "some networking problem". In some forums they talk about timeouts.
An Iphone works fine.
The same settings on the WM6-mobiles worked some time ago as well.

Any idea where you have to change settings?

Thanks
Christian