Bench rig at school, TIA Portal V17, an S7-1200 CPU 1214C with a KTP700 Basic PN. It was all working last week, the panel showed the tank level and the pump state out of DB1.

Then I read a tutorial saying data blocks should be optimized, so I ticked Optimized block access on DB1, compiled and downloaded the PLC. Now every field on the panel shows ### instead of a number, though it still says connected and the icon's green.

I downloaded the HMI again and restarted the panel, no change there, and the connection in Devices and networks still points at the right CPU. Then I dragged the level out of DB1 into a brand new HMI tag and that one shows a value.

Screenshot of the tag table attached. Why would the old tags break and the new one work, am I missing something obvious?