from time to time CT does not open reference translation memory even though it was created on CT. What's more an attempt to open it blocks detached window function. What is the reason for this?
It may happen when you have already opened a TM with the same name. If this is the case indeed, make sure that that TM name is unique. Otherwise, please submit a support ticket with the TM attached to reproduce this issue.
It may the case because after joining the project TM and the external one it started to work.