Custom-field validator fails in workflow transition

[Lena Wiethaus]

I added several validators in a workflow transition with a defined screen. Only one of them is giving out an error. We start in the status "open" and as per default value "Espera intern" is set. If the issue is transitioned to the status "waiting in support", the screen pops up and all data required by the validator has to be entered. In this case also a custom-field "customer" which a single-choice selection list. If a value has been pre-set the transition fails with the error, that no value has been set for this custom-field. All other custom-fields are working well, also vor single-choice selection list.