Hmm, sounds like some sort of corruption may be at fault here. Checksum implies that one or more files aren't the size that the program expects them to be. I think it's worth replacing all current Mintcoin files with new ones (except wallet.dat ofc)

Give this a shot:
1. Close down your wallet
2. Backup the wallet.dat file somewhere safe.
3. Delete all Mintcoin files on your PC - the QT.exe folder as well as the data in %appdata%\roaming\Mintcoin\... (assuming you're on Windows)
4. Download a new copy of the Mintcoin wallet 1.5
5. Run the exe, wait for it to open and then close it again.
6. Copy your backed-up wallet.dat into the %appdata% folder
7. Restart your client and see if it syncs.

As the coins haven't moved this won't reset their age, as well as ruling out 99% of files potentially corrupted.