Mystery "unable to extend" error

This is an old 8.1.7 installation on Linux, which suffers from the "2 GB per file" problem. A file upload started giving "ORA-01691: unable to extend lob segment" errors so I added another data file to the tablespace, but inexplicably (to me) the errors continue. 

There is now plenty of free space in the tablespace:
Tablespace Name Total Bytes Used Free %Used ---------------- ------------------ --------------- ---------------- ------ 
USER_LOB          17,112,760,320  14,613,618,688    2,499,141,632   85.4

And yet..
error in `OCILobWrite ()': ORA-01691: unable to extend lob segment USER.SYS_LOB0000041854C00017$$ by 599124 in tablespace USER_LOB 

(where I have replaced the actual user name with USER)
I have never seen this before; adding another data file has always been the solution in the past. I don't even know where to look next. Any suggestions? 

thanks,

janine

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Janine Sisk
President/CEO of furfly, LLC
503-693-6407

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