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.