Problem

When PM edits task (where he is not workflow step user) nextsegments HTTP request fails with error 500 and nothing appears in the "Matches" panel. 
PHP error log entry:
ERROR Error: E9999 - Call to a member function isSegmentrangedTaskForStep() on null
  in  /application/modules/editor/src/Segment/ActionAssert/Permission/Asserts/SegmentIsEditableAssert.php&line=82 
  User: manager1 (Project Manager) ({65751ea9-6834-444f-bd3a-2de95a335675}) 
  Request: POST http://translate5.local/editor/taskid/2493/segment/nextsegments

Solution

Handle PM user properly.