Hmm, if you were only using 0.5.18 then it probably wouldn't be a format thing.

It is a pretty big wallet you have there and the error message seems to be related to the wallet size:
com.google.protobuf.InvalidProtocolBufferException: Protocol message was too large.  May be malicious.  Use CodedInputStream.setSizeLimit() to increase the size limit.
   
Classic isn't being actively developed now though I will raise this as a bug.
We don't produce releases of Classic very often now so it's not going to get fixed quickly.

I would recommend moving your wallet to blockchain.info using:
https://multibit.org/en/help/v0.5/help_movingAWallet.html

If you use an older backup of your wallet you should be able to open it and export your private keys.

edit: https://github.com/bitcoin-solutions/multibit/issues/744