1.) Go to your blockland folder
2.) Move Key.dat (This removes you key having this issue related to keys.) to some where safe just in case
3.) Open Blockland. It should be in demo mode. If it runs go to step 4. This is to check if it isn't a problem with BL itself
4.) Right click your key.dat File, properties.
5.) Attributes: MAke sure "Read only" isn't checked. If it is, check it off and move it back to your BL folder.
6.)Run BL
7.)???
8.) Profit
Also, I did a test. I Made mine read-only on purpose. I got the same message. Then I turned off read-only. It got rid of it.