Hi,
We had used addCustomView function to filter lookups in our email entity. However while changing the entity inside the Lookup dialog we were getting the above error.
We read in the forum that installing the Update Rollup 10 should fix the issue for the multi-entity lookup. We had Update Rollup 6 in our system and as suggested, installing UR 10 resolved the issue.
Check out the helpful link
http://social.microsoft.com/Forums/en/crmdevelopment/thread/9d3d60fb-106e-4e4c-b846-295cea4d800e
Hope it helps.







