Interesting observation, which I've managed to reproduce, i.e. if the X server (i.e. 'Xvesa') and 'fxce4' WM have been started by any other user than 'tc' the command completion with the tab key does not seem to work. The terminal seems to respond instead with a "visual bell" (i.e. a brief flashing of the terminal window).
I've now made some other "discoveries" that I can't explain either:
(1) If one uses in the same situation as above the key combination of 'Ctrl-i' (which should be the equivalent of the 'tab' key) the file name completion is working (note: for the first time it appears to be required to enter the 'Ctrl-i' twice for it to work, but that is no different when using the tab key).
(2) Now changing the "experiment" somewhat, by starting the X server and the WM as user 'tc', and then changing the user in a terminal (e.g. via sudo su malic). In this case the completion with the tab key appeared to working as it should.
After a bit of toying around with 'strace' of the shell process in question it became clear that hitting the tab key or entering 'Ctrl-i' were not seen as the same thing. Eventually I decided to take it a step further and ran both cases in two different QEMU VMs to get a better view over the differences: When starting 'Xfce4' for the very first time user 'malic' gets asked in the "Welcome to the first start of the panel" pop-up window whether a default config should be used. For user 'tc' this question is somehow already been taken care of.
Even after taking snapshots of the home directories for both of these cases and comparing them I have not (yet) been able to pinpoint where the source of the problem is to be found. Its getting late here and I'll need to now leave it at that. Clearly there are (small) differences between the config files found in the home directories for both cases. But as I don't know how all the config files are getting populated I reckon that leaves it to bmarkus to figure it out (if he wants to spend his effort on this question).