I have reproduced this with 2 sprints of rather large size. Sprint 1 has 225 items (all bugs), sprint 2 has 125 items (stories). The workflow for bugs is the standard JIRA, the workflow for the stories us custom made.

When attempting to remove an item from a sprint, either through the plan mode or work mode, the sytem times out with "read time out" error. This happens for any item in the sprint. Items have been manually transitioned through the workflow, as well as bulk transitioned.
I have only reproduced this within one project so far.

The item is not removed from the sprint.

I have reproduced this with Chrome Version 23.0.1271.95 m and IE8
System is runing ubuntu 32 bit, a turnkey appliance.