[Users] Problems importing messages from mbox

[IMacaulay]

Folks, I'm currently trying to move to Claws mail, but cannot seem to
import my old messages.  My 'previous' mail client is Opera 12.16.  I
have generated mbox files using Opera and am trying to import these
into Claws, but this fails after about 10000 messages (varies between
10000 and 10500).  The mbox file, itself is about 1.3 Gb and contains
considerably more than 10000 messages.

I have tried importing a different (smaller) mbox file after the
previous one failed, and that, too, failed, as though what was
stopping me was a fundamental size limit in Claws.

Could anyone possibly suggest what might be giving rise to the
problem, what to try changing, or if there is indeed some limit in
Claws that means I cannot import such a file?

Thanks,

Ignatius.