Shared clipboard is not working in GUI mode

[VBoxer1]

How to reproduce:

1. Set the bidirectional clipboard mode.
2. Run the guest for a while or pause and restore it. (I haven't yet found out the trigger for sure).
3. Copy a text from the guest into the clipboard.
4. Paste the text from the clipboard into the host or another guest.

Expected result: The clipboard text is pasted into the host/other guest.

Actual result: The clipboard text is not pasted into the host/other guest.

Unlike bug #15986 the problem occurs in GUI mode. The other bugs referred in bug #15986 don't match here. It only occurs in a Debian Linux guest, not in a Windows 7 guest. Copy and paste within each guest works fine. After restarting the guest the clipboard works fine. I noticed it first in version 5.1.6, but the problem persists in version 5.1.10.

The clipboard related entries in VBox.log are:

00:00:05.989716 Shared clipboard service loaded
00:00:05.989747 Shared clipboard mode: Bidirectional
...
00:00:18.768979 HGCM: restoring [VBoxSharedClipboard]

[philip]

Related: Bug #15782 -- I'm unable to use the clipboard in 5.1.x but it works fine in 5.0.x, so I keep reverting back to 5.0.x after giving newest 5.1.x releases a try.

[philip]

It appears this bug is related to the type of content that is being copy-n-pasted. For details, see Bugs #15782 and #16242.