This problem bedeviled us for almost two weeks now, and now that I've found the cause, hopefully I can spare some lucky soul the pain and agony of all the troubleshooting we went through here.  Here's our setup:

Interplay 1.2.1

ISIS 1.4

Symphony 1.8.1/Adrenaline 2.8.1

2 separate Interplay/ISIS clusters, visible to each other

Our media management person reported that the edit supervisors could not do a "Send to Workgroup" in Access to move media between our two clusters.  Once you selected "Send to Workgroup" and the window opened, they were able to select the destination folder in the other cluster's Interplay structure, but the "Workspace" drop-down would stay greyed out, not allowing them to select a destination workspace for the media itself.  It turns out that both the source and destination ISIS clusters must have the same password set for that particular user's login, which must match the password they logged into Interplay with, otherwise that drop-down box won't populate.  We import our user info from Active Directory for our Interplay clusters, so it turned out that the edit sups had in all three cases done their regular password update on one ISIS cluster, but not the other.  Once they changed their passwords to match, the "Send to" went through just fine.

Hope this helps somebody out there.....