Version:           1.5.1 (using KDE 3.5.0, Debian Package 4:3.5.0-3 (testing/unstable))
Compiler:          Target: i486-linux-gnu
OS:                Linux (i686) release 2.6.12-1-686

If I have created a blank assotiation between two entities and then assigned some properties to that assotiation (name, multiplicities, roles) that I can change those properties in 'Properties' dialog, but I cannot make it blank again by removing them, so I have to delete the whole assotiation and to create a new one.
The same thing if I try to delete some property label on the diagram by selecting it and pressing "Del". 
Sanity check:
1. Create new ER diagram.
2. Create two entities (say A and B).
3. Create a simple assotiation between them. 
4. Assign muiltplicity (say 1-1) and roles (say foo-bar) to that assotiation.
5. Open 'Properties' dialog again and remove all text in 'Roles' tab. Press 'Apply'.
6. Look, nothing has changed. 
7. OK, now close the dialog and select "+foo" label on the diagram.
8. Press "Del". Open 'Properties' dialog and look that nothing has changed again.