Your system log is full of 3rd party related errors.
e.g. when viewing a topic, one of the hooks results in an exception for guests resulting in search engines and guests getting a "Sorry something went wrong" error instead of the topic.
BTW Is it working now for you? I've just tested it and it works fine for me. Have you tested it with another account?
If it's still broken, I would suggest to disable all the PM related 3rd party applications and plugins, rebuild the cache and see if it works. If not, take a look t the full error which is returned, it should include a list of all 3rd party apps and plugins which are doing something on this page.