AWS DMS (Data Migration Service): Connection refused during pregmiration assessment run

Question

I have a "Database Migration Task", the source is S3 (CSV files), and target is Postgres. I have tested each Endpoint connection (source and target), and they work fine.

Also, the DM Task use a custom "Replication instance". During the pre-migration assessment run, when checking for "Unsupported data types" with the source, it got canceled. Last failure message said "Connection refused.".

I tried to read the assessment output, but I can't find anything useful.