When opening your Lockbox 3.x database and you receive the error "Database file is an invalid TinyDB.", this means your Lockbox database has become corrupted.  The most common reason why this happened is your system may have had a hard-reboot or a system malfunction while the database was opened. 

To correct this, you will need to restore your database from a backup. There is no way to repair a Lockbox database file once it has been corrupted. Here is what you need to do:

Step 1:

Locate your exiting Lockbox database in one of these location and remove all copies of files ending in .PWK.
1. C:\Program Files\CoffeeCup Software\CoffeeCup Lockbox folder.
2. C:\Program Files (x86)\CoffeeCup Software\ CoffeeCup Lockbox folder.
3. C:\Users\[account-name]\AppData\Local\VirtualStore\Program Files\CoffeeCup Software\CoffeeCup Lockbox folder.
4. C:\Users\[account-name]\AppData\Local\VirtualStore\Program Files (x86)\CoffeeCup Software\ CoffeeCup Lockbox folder.

Step 2:

Locate your backup file of your Lockbox databases and copy them back to the corresponding folders above.

Step 3:

Open Lockbox and with any luck, the error will have disappeared. 
NOTE: This would be a good time to consider upgrading to the latest Lockbox 6 if you have not already done so.  Version 6.x has many new features and enhancements to help prevent this sort of error.