Comment 4 for bug 985986

Apparently you ran out of memory right before the swap partition was enabled by partman.
Was anything running in that session besides ubiquity?

Can you reproduce this issue every time around when you have 600MB of RAM?

If so, the problem is likely on Lubuntu's side where something is running eating your memory in the live session.
You may then want to try the Install only mode that doesn't start a full live session and should be working just fine with that amount of memory.