While working fine for almost a year now (since I've installed CyanogenMod on my device), a couple of days ago synchronisation stopped all of a sudden: Network indicators turned white (no longer green on Gingerbread / blue on ICS and up), which indicates the device is not "in-sync" with the Google servers.
First I thought it might be some Google Server outage (as all of us know them), I waited a couple of days. But it didn't solve up within a few days, so I decided to take a closer look.
logcat then showed: Always when trying to sync, the network connection timed out.
Further side-effects: Google Playstore app did no longer open ("network timeout"), Google Talk insisted on a "username/password failure".
That's where I got stuck: What to do? Reboot didn't help, shutdown/restart didn't help either, nor did clearing cache/data from the contactssync app. I couldn't log off GTalk (which would be the next logical step), as GTalk immediately insisted on that username/password error.