I use bordermanager 3.7 SP3 on NW6.0 SP3 for serveral years without any

Suddenly, the timesync gives problems by not staying in sync.
Result: "Proxy bordermanager : internal error" page on the client browsers.

The onely time that happened is that our ISP had some problems with his
own nameservers.

Our Bordermanager uses 3 nameservers:
1. Internal DNS
2. DNS of ISP
3. DNS of ISP

The timesync server for all servers is the internal server with the
master replica. The server with BM has also a full R/W replica.

Like I said, everything went well for several years but suddenly ....
Can anyone help me ???