How many entries are you trying to move? One possibility is that the client session is timing out due to inactivity, but it could also just be the amount of data, or a problem with one of the entries.

Moving files between repositories is more like an export/import, so there's more going on behind the scenes (i.e., it's actually creating and importing a briefcase to perform the "copy and paste") and you can run into issues if you're trying to move too much at once.

Whenever we've done large moves from one repository to another, we break it up into smaller more manageable chunks; Workflow's Replicate Entries activity can be a more efficient method.