first we need to know more about the error - please play again and when that screen happens, press F8 and give us a screenshot of the console output.
That said, the core version and the plugin versions need to be on the same level - if a plugin is written for 1.5.1 then you should stay at 1.5.1, if it is written for 1.6.1 then you should update to 1.6.1
We would need a screenshot of your plugin manager with all plugins listed to help you with that decision.
That said, please make the following test:
Start playtest, start a new game in the playtest (DO NOT CONTINUE), save on the first map - does the same error happen or not?