Any ideas why I'd be getting a "memory allocation failed" exception when signing a transaction imported from the qr code - importing from the sd card works fine.
That usually happens when the PSBT you're trying to sign is quite large/complex. What version firmware are you running? If less than v2.0.6, please update to that as we've made quite a few quality of life improvements in it.
We're also about to release a beta for v2.0.7 that has a completely reworked transaction parsing memory approach that will allow for signing *much* larger transactions via QR, but we do still recommend microSD for larger transactions as the process is much faster and easier that way.
it's 2.0.6. Yes, the psbt is likely quite big as it's 3/6 multisig.