OK, so I used the F2 to enable the debug screen, and this really helped.
First the debug complained about the firewall, so I deleted and reconfigured the Firewall settings.
Second the Debug complained about DHCP Exclusions. Interestingly, the only DHCP config and Exclusion where on the Voice VLAN, but I deleted them anyway.
After that the Data VLAN IP changed succeeded, but the US500 had one more surprise in store.
I could ping the new IP, but couldn't access the UC500 with CCA.
I had to get into CCA using the Voice LAN, then delete and reconfigure the firewall settings once more.
The everything worked as it should.