The Urban Arrow cargo bike has an integrated frame lock, which prevents the rear wheel from rotating when locked. The lock is attached to the frame using lock rubbers, which prevent a spoke from breaking if the bike is moved when locked. Instead, the lock rubber selflessly sacrifices itself, and after a moving memorial service you'll need a replacement.
In the UK, where the Urban Arrow is uncommon, it is not cheap or fast to obtain an official replacement lock rubber. However, this part is not specific to this bike. The general term is an anti-vibration mount bobbin isolator, and you can search for some variation of this term on ebay. It has a 15mm body, 15mm stem and M5 thread. The exact part is on AliExpress, labelled "15*15mm Body M5*15mm Male Female Rubber Anti Vibration Mount Bobbin Isolator".
I ended up buying 2 on ebay for about £7, and they only took a few days to arrive.