A serious bug has been found in the recent 3.11.0 release… newly encrypting databases doesn’t work (#1732), instead displaying an error message. 😦
Existing databases - encrypted or not - can be opened without problem. The problem only occurs when attempting to newly encrypt a database.
The links on our download page have been reverted back to the prior stable release (3.10.1) for now, while we look into this.
There will probably be a 3.11.1 release in the near future, with the problem solved. 😄