What it means Ground fault found on output phase U.
Likely causes
Insulation failure on U lead or winding
Checks, in order
Megger U lead and winding
Source
Source (reworded, not copied): Rockwell Automation, PowerFlex 520-Series AC Drives User Manual, publication 520-UM001O-EN-E (September 2025), Chapter 4 Troubleshooting. Official manual (PDF). Fault numbers and names can differ between firmware versions.