For anyone with similar issue, this can happen when
1. You have a contract for multiple tourists.
2. You complete the contract for one of the tourist, so that Kerbal tourist appears to disappear from your game.
3. You recruit a Kerbal with the same name as the tourist that no longer exists and assign them to a mission.
4. As soon as you fully complete tourist contract, you get the roster verification error.
It seems the game gets confused because one of the tourists appears to be assigned to a mission so it isn't safe to continue.
To resolve, head into the relevant .sfs file prior to contract being completed (please take a back up!), find the tourist in the contracts section (just search for their first name) and change their first name to something unique - including in the contract parameters section. Do not change the crew member of the same name's first name! Reload your game, complete the contract and everything should still be dandy.
I got this error in v1.8 so it's probably still a live bug. I gave up trying to figure out how to log it with Squad but figured I'd log solution here as this is where my Googling brought me.