I am experiencing the exact same problem. On Thursday, my BF and I both upgraded to 1.5, and he could join my farm with no issue. However, we started getting the communication errors on Saturday. Now every time my BF joins, it errors out.
I receive the screen "The software was closed because an error occurred." We've tried all of the same things to troubleshoot, but with no luck.
I made a new farm, and he can join that one. I can also join his farms. It appears that the only issue is with the one (and sadly most complete) save file.