This issue has been reported before but I did not notice a resolution in that thread. Applying the fix suggested here did not work. This post ended up being the most comprehensive review of the issue. It suggests this command to fix the tab key behavior in XFCE:

$ xfconf-query -c xfce4-keyboard-shortcuts -p /xfwm4/custom/'<'Super'>'Tab -r

This worked for me.

On Sat, May 7, 2016 at 3:10 PM, M.Rule <mrule7404@gmail.com> wrote:

It seems to affect all applications. Cannot insert tab in text, cannot tab complete in terminal. Alt+tab, control+tab, and command+tab all still work and register with the xserver.

Has anyone seen this / solved this before?

Configuration:

OSX 10.10.5 Yosemite on a late-2008 aluminum macbook
Client: X2Go Client v. 4.0.5.1 (Qt - 4.8.7)
Server: Ubuntu studio, not sure what versions at the moment

Best,
Michael.