No, a mount.xtreemfs (aka xtfs_mount) crash shouldn't affect data,
since the client doesn't cache anything by default.
It would be helpful for us if you would run the mount command with -f
(to put it in the foreground) and -d ERR to log any errors to stderr
(e.g. > log-file-name 2>&1). It might also be useful to output core
dumps (ulimit -c unlimited) and use gdb to see why the client crashed
(assuming it crashed rather than simply becoming unresponsive).
Minor