I would start at the github repo and check if that issue has been documented.
If yes, follow the instructions. If not, check your dns since 502 often came from dns in my case.
If both doesnt reveal anything, you could open an issue in the repo and post your (sanitized) logs and wait for answers.
The error suggests a problem with the lemmy-lemmy-ui-1 container. Maybe it needs an update or has pulled a wrong update. When did you update last? Did you try restarting the stack?