There is a column called "Team Member" available in the New UI's "Tasks" tab. Due to a limitation in the number of characters that a particular cell can hold, tasks with large numbers of resources assigned against them, throw an error with code API-1019 in this tab.
This error in the UI is accompanied with an error such as the one reproduced below in the app-ca.log:
ERROR 2018-08-29 05:27:17,113 [http-nio-14001-exec-1] ppm.rest (clarity:admin:5190042__FFACD205-5A01-4B6D-B9FF-BF65C7EFE8D0:PPM_REST_API) ODFObjectMarshaller :: attribute: assigned_resources has an invalid lookup value: [T5003000, T5003009, T5003099, T5003999, T5004000, T5004001, T5004002, T5004003, T5004004, T5004005, T5004006, T5004007, T5004008, T5003100, T5004009
T5003977, T5003978, T5003097, T5003979, T5003980, T5003981, T5003982, T5003983, T5003984, T5003985, T5003986, T5003987, T5003988, T5003098, T5003989, T5003990, T5003991, T5003992, T5003993, T5003994, T5003995, T5003996, T5003997, T5003998]
This issue has been logged as a defect with ID DE43682.
DE43682 has been addressed as of PPM 15.5.1.
Remove the 'Team Member' column from the Task list view. This can be done by expanding the Column Panel on the right hand side of the screen and deselecting the check box next to the 'Team Member' item.