You see a BlockStoreException very occasionally when MultiBit Classic starts. It prevents you synchronising with the Bitcoin network.
You can fix this problem by doing the following:
Find where MultiBit Classic is storing your user data - see the help topic Troubleshooting
Shut down MultiBit Classic
Go to your MultiBit Classic user data directory - worked out in step 1 - and delete the file multibit.spvchain
Start up MultiBit Classic again