It's a known issue: on 028stab033 checkpointing is broken! It's fixed in git already with the following patch: http://git.openvz.org/?p=linux-2.6.18-openvz;a=commit;h=650723f83a8685111fb3dee7073454861c446193
So, in order to use checkpointing you can:
1) use 031 kernel before 034 is released
2) compile fresh kernel from git