ORA-39095: Dump file space has been exhausted: Unable to allocate 8192 bytes
Looks like you ran out of disk space
Just Adding one thing,You can use the Parallel option and use different directory to distribute your dump file at multiple FS .
Can you share the export command and the available space for /backup mount point ?
It looks the /backup mount point free space is less then the exported dump file size.
When you say the export works for another schema in the same database are you using the same syntax and would the export file sizes be near enough the same?
Have you seen this metalink note?
DataPump Export (EXPDP) Error ORA-39095 Dump File Space Has Been Exhausted [ID 559692.1]
It could also possibly be a ulimit setting or filesystem space issue.