You need to enter the issue number in the edit box on the left side of the add button before you click add.

ok. It was a "user problem". I was not expecting that behaviour (it assumes you know by heart the number of all your issues).

thank you!

Typically I do one of the following:

1. Have the issue open in a different browser tab.
2. Have the issue in the recently visited list just under the top menu.
3. Remember the number based on a lookup just before adding the relationship.