The message was relatively clear (for a XAPI error 😛 😞
This operation cannot be performed because the specified VDI could not be found on the storage substrate
sr: 28a8b360-7b1b-58ca-81b6-f907c164ec38 (XCP-ng Tools)
vdi: b1a7a49a-f17c-42ac-af73-4429eb57b07a (Old version of guest-tools.iso)
This means the VM can't boot with this current CD because it's not there anymore. I suppose you updated your host without ejecting all CDs before. So the old tools CD was removed for the new one, and your VM kept it. So when you tried to boot, it couldn't because it couldn't find the old ISO.