Tmux borders displayed as x q instead of lines?-open source projects kriskowal/q
There is some mismatch between your terminal emulator and the terminfo database entry being used by tmux (the one named by the TERM environment variable when you start/attach to a tmux server).
Per the VT100 User Guide, Table 3-9: Special Graphics Characters, when the “special graphics set” is selected, x is used to draw the “Vertical bar” and q is used to draw “Horizontal line – Scan 5”.
Under terminfo, the VT100 special graphics characters are available as a part of the Alternate Character Set functionality; see the “Line Graphics” section of the terminfo(5) man page.
Probably (on your Debian server) the effective terminfo database entry indicates that ACS is available, but your terminal emulator is not actually responding to the specified control sequences.
The tmux CHANGES file indicates that some terminal emulators (e.g. Putty) do not respect the ACS control sequences when they are in UTF-8 mode. Thus, tmux 1.4 has a change that makes it always use UTF-8 characters instead of ACS sequences when the attaching client specifies that it can handle UTF-8 (i.e. when attaching, -u was given or UTF-8 is present in LC_ALL, LC_CTYPE or LANG; the utf8 window option is about what tmux should expect from the programs it runs, not what it can send to the attached client).
Debian “squeeze” only includes tmux 1.3, so your tmux probably does not have the “prefer UTF-8 line drawing” feature (unless it pulls from a backports source).
If you can not fix your terminal emulator nor upgrade to at least tmux 1.4, then you might be able to use tmux’s terminal-overrides option to unset the ACS-related capabilities so that tmux will fall back to ASCII line drawing. In your .tmux.conf (on the Debian system):
set-option -ga terminal-overrides ',*:enacs@:smacs@:rmacs@:acsc@'