These may be possible causes, however that has not been my experience.
- You try to use your Blockland install on another computer.
As long as you don't use them both at the same time, I have had NO problems at all. Unless you copy the key.dat from one computer to another instead of re-entering your key on the 2nd computer. I believe this is so that you cannot just steal a key.dat file from somebody and use it on your own computer.
- You try to use Blockland using a different internet connection (wired instead of wireless maybe)
Changes all the time for me, never had a problem.
- Your key.dat file is not writeable (C:\Blockland\key.dat)
This just gives you a warning.
Again, this is just my experience.