This issue is related to the story #9090.

When you have a computed field that need to be updated (but already have a correct target field) you can't edit the edit the field in the field usage area. A fatal error is thrown and the form is not displayed.

Trace:
Fatal error: Call to private method Tracker_FormElement_Field_Computed::useFastCompute() from context 'Tuleap\Tracker\FormElement\View\Admin\Field\Computed' in /usr/share/tuleap/plugins/tracker/include/Tracker/FormElement/View/Admin/Field/Computed.class.php on line 88
Call Stack
# Time Memory Function Location
1 0.0000 634656 {main}( ) ../index.php:0
2 0.0317 4281784 TrackerManager->process( ) ../index.php:33
3 0.0318 4282040 TrackerManager->processSubElement( ) ../TrackerManager.class.php:127
4 0.0459 7042784 Tracker->process( ) ../TrackerManager.class.php:110
5 0.0483 7703104 Tracker_FormElement->process( ) ../Tracker.class.php:597
6 0.0485 7703696 Tracker_FormElement->displayAdminFormElement( ) ../Tracker_FormElement.class.php:203
7 0.0501 8031880 Tracker_FormElement_View_Admin_UpdateVisitor->display( ) ../Tracker_FormElement.class.php:377
8 0.0501 8032128 Tracker_FormElement_View_Admin_UpdateVisitor->fetchForm( ) ../UpdateVisitor.class.php:56
9 0.0582 9634496 Tracker_FormElement_View_Admin_Field->fetchAdminSpecificProperties( ) ../UpdateVisitor.class.php:35
10 0.0582 9634776 Tracker_FormElement_View_Admin->fetchAdminSpecificProperties( ) ../Field.class.php:46
11 0.0585 9636400 Tuleap\Tracker\FormElement\View\Admin\Field\Computed->fetchAdminSpecificProperty( ) ../Admin.class.php:202
12 0.0585 9636480 Tuleap\Tracker\FormElement\View\Admin\Field\Computed->isMigrated( ) ../Computed.class.php:59