If I boot back into kernel version 3.13.0-96, the problem disappears.
With the prior kernel version 3.13.0-98 I have the same problem as with version 3.13.0-100 (but I think I just did not notice that this problem also occured with this prior version).
On my desktop machine, which runs the same Ubuntu version 14.04 but configures the network settings with the Network-Manager (instead of the setting "iface eth0 inet6 auto" in "/etc/network/interfaces"), this problem does NOT occur.
If I boot back into kernel version 3.13.0-96, the problem disappears.
With the prior kernel version 3.13.0-98 I have the same problem as with version 3.13.0-100 (but I think I just did not notice that this problem also occured with this prior version).
On my desktop machine, which runs the same Ubuntu version 14.04 but configures the network settings with the Network-Manager (instead of the setting "iface eth0 inet6 auto" in "/etc/network/ interfaces" ), this problem does NOT occur.