Doing a Backup ONLINE, this ends with "Failure 2843:No more room in database."
In that database logs are written?
When you do an online backup any inserts/updates/deletes on a table while the backup is running are recorded in a special subtable, thus the tablesize might grow leading to 2843.
Every table in Teradata consists of subtables (e.g. table header, primary data, fallback data, secondary indices), thus it's using perm space in the table's database.