the problem i faced is similar, using ESXi 5.0.0 (Build 914586) with vSphere Client 5.0.0 Build 913577.
attached the log to this post.
the log shows that there was some connection problem at the following timings: 17:13:41:xxx and 17:15:02:xxx
i'm pretty sure it is not connection problem as i've got other esxi connected to the same setup, and i've swapped the cables, ports, etc.. could it be due to some settings problem?