Resizing problem with linux guests

[mskov]

Starting from VirtualBox 5.1.0 resizing of linux guests doesn't work well anymore. Sometimes it works, sometimes it takes a few seconds to resize and sometimes the screen doesn't resize at all (that happens on the same running instance of a VM, without restarting it). We observed this problem on VirtualBox 5.1.0, 5.1.2 and 5.1.4 (with the corresponding version of guest additions installed). The problem occurred both for the Ubuntu 15.04 guest and for a Debian 8 guest.

After every (!) restart of the VM I see the following messages in the system log:

Sep 01 18:40:36 sirrix-VirtualBox vboxadd.sh[654]: Starting the VirtualBox Guest Additions.
Sep 01 18:40:36 sirrix-VirtualBox vboxadd-x11.sh[706]: Installing the Window System drivers.
Sep 01 18:40:36 sirrix-VirtualBox vboxadd-x11.sh[714]: Installing X.Org Server 1.17 modules.
Sep 01 18:40:37 sirrix-VirtualBox vboxadd-x11.sh[727]: Setting up the Window System to use the Guest Additions.
Sep 01 18:40:37 sirrix-VirtualBox vboxadd[533]: You may need to restart the the Window System (or just restart the guest system)
Sep 01 18:40:37 sirrix-VirtualBox vboxadd[533]: to enable the Guest Additions.
Sep 01 18:40:37 sirrix-VirtualBox vboxadd-x11.sh[735]: Installing graphics libraries and desktop services components.
Sep 01 18:40:37 sirrix-VirtualBox vboxadd-x11.sh[748]: Window system drivers installed.
Sep 01 18:40:37 sirrix-VirtualBox vboxadd.sh[794]: .
Sep 01 18:40:37 sirrix-VirtualBox systemd[1]: Started vboxadd.service.

The VBox.log from 5.1.4 with an Ubuntu guest (guest additions 5.1.4) is attached.

If we revert the additions to 5.0.20 - all works fine (with virtualbox 5.1.4).

[mskov]

Well, actually with additions 5.0.20 the resizing problem also occurs sometimes for me (with VirtualBox 5.1.4). But in that case I don't get those weird messages in the system log.