ReadyNAS 214 data corruption issue

I am seeing a very strange behaviour that results in corrupted files. My NAS (ReadyNAS 214) has been stable for years, until recently I found a single file had an error revealed via the logs following a scrub. I recoved that from a backup.

Over the past two days, I have noticed various newly written files appear fine, but after 'a bit of time', they become corrupted. Yes, weird I know. This has happened with edited Excel (xlsx) and non-edited .jpg and PDF files. Not all files going to the NAS are affected.

To isolate the issue:

• I scanned my PC in case of a virus. Nothing found.
• Created a checksum file, then copied the files to the NAS. Reread the checksums and all appears good.
• A variable time later, re-check checksums. Results bad.
• The time to going bad is immediate if I re-copy a file within the NAS folder, i.e. CTRL-C / CTRL-V, then check the checksums, and both the original and copy become corrupted.
• I initially noticed this issue with photos as previews no longer showed, but repeated with xls and pdf.
• I shutdown my PC in case it was the cause in some way, e.g. undetected virus.
• I then tried from my wifes PC and was able to recreate the corrupting of files, almost at will.
• I'm now convinced it's a NAS problem, rather than a PC problem, but maybe I am wrong!

So, I dived in to the logs and found quite a few lines like the following line in kernel.log:

Aug 26 17:54:53 NETDISK kernel: BTRFS error (device md124): bdev /dev/md127 errs: wr 95, rd 333, flush 0, corrupt 0, gen 0

Which of course looks bad, and is probably why I am getting corrupted files.

Any ideas how I identify the root cause, and fix this, as right now I cannot trust anything writen to the NAS?

Luckily, I think I caught this early and have other copies of the files, or can re-create any lost data easily.