Try to edit/define Message Composer's Regular Expressions. If you write an invalid character like '(','[','\' etc. then a validation message appears. For example: "Unclosed group near index 1" for '('. If you correct the regular expression then the message is still present and it is imposible to save correct value unless you reopen and re-fill the dialog. In case of definition of Regular Expression for a new binding you have to create the binding from the beginning because Finish button is disabled. The problem occurs only in Properties dialog and not in Properties view.