Not really a surprise…, after some edits I have, with that project, already up to 13 GB RAM used. 
(Of course with some other applications like browser (10 tabs)…)
It used the SWAP, but SWAP is slowly, so Shotcut stopped the export (IMO), which was (is really) good, because it slowed down my whole system (“swimming mouse pointer”, a.s.o.). (Linux Mint here… 
)
Yes, after closing Shotcut and restart, export as usual.
'Til then…