Hi Nicola,
323 GB free space and you get an ORA error like ORA-01652 / ORA-01653? An extra data file fixes the issue temporarily?
Exadata, ASM and AU size .. this sounds like the "classical" extent alignment / AU boundaries issue. It is described by Randolf Geist in one of his blog posts: ASM AU Size And LMT AUTOALLOCATE
Regards
Stefan