Export of symbols derived from the central library is not available as you note. But if you can still see the symbol in the Symbol View then you are still looking at the central library, so I don't really understand that statement.
Anyway, one thing you could try it to copy the symbol file out of the library tree and then try importing this file back in to see if it fixes the issue. The symbol is just an ASCII file stored in the SymbolLibs folder tree. If the file is corrupted then you will need to recreate it or find a backup if you have made one.
thanks for your quick reply.
Actually I could initially see the border symbol listed in the symbol View but then it disappeared from the list (I guess the view had to sync with the central library?). Anyway I'm still not sure why the border symbol got corrupted in the central library in the first place.
For future reference, I managed to copy the symbol from Designer into the Symbol Editor exporting/importing as DXF.
Hope this helps if somebody has the same problem.