I have an 2820n IPPBX that seems to loos all its settings occasionally on reboot.

It is not on the latest firmware so I am going to update.

Looking in the release notes for 3.5.5 it says:

23. Support backup config data mechanism to prevent setting became factory
default after reboot.

Is this Chinglish for saying this issue is fixed?

I would say so. My IPPBX 2820 started doing the same yesterday. Upgraded fireware and it seems fine now.
Odd thing is I can't now get my Draytel account to register. Hopefully I've done something wrong and it's not the new firmware.

I upgraded it yesterday and had to reconfig the whole thing as the backup configs I had totally killed it when reloaded on every time!

I wouldn't mind so much if it is mine, but it is a customers and as I recommend it (based on the performance of their other products) to them it is making me look bad!

3.5.5. is a major/significant upgrade, so starting a afresh is probably a good, if inconvenient, idea. Shouldn't normally be required.