This article applies to PRTG Network Monitor 18 or later

Oracle Tablespace Sensor and Negative Values

If your Oracle Tablespace sensor is showing negative values and is therefore in a Down status, this could be because the tablespace has been overprovisioned. This means that the actual file size of the tablespace exceeds the defined maximum size of the tablespace, which was defined when it was initially created.

The Oracle Tablespace sensor does not support overprovisioning.

For the sensor to properly monitor a tablespace, the file size on-disk must not exceed the value in DBA_DATA_FILES.MAXBYTES. If the file size on-disk exceeds this value, then the sensor reports negative values indicating that the file contains more data than defined in the database metadata.

So, the sensor is working exactly as intended. It as not a issue of PRTG.

Workaround

The only workaround to avoid such errors is to delete the sensors that are monitoring overprovisioned tablespaces. If you cannot delete them or still want to be able to see data for overprovisioned tablespaces without receiving errors for Down sensors, then you can simply acknowledge the alarms.