OK, I am up and running again with 880.4 and most of my 879.9 settings. Through a meticulous process of manually merging tws.xml files, I've concluded that: - The incompatibility is in the WindowConfiguration settings. - In particular, the following WindowConfigurations in my 879.9 settings apparently cause TWS 880.4 to throw a null pointer exception and hang: Click Trader|0 (I assume these are the BookTrader windows I had open at some point) Click Trader|1 Deep Book Window|0 (I assume these are the Level 2 windows I had open at some point) Deep Book Window|1 I am wondering if the bug is really -- right now I don't have any Level2 subscriptions whereas at one point I did. So I wonder if the bug lies in trying to load those configurations given that I don't have Level2 subs right now (even though those windows are closed.) I will add this info to my ticket with CS -- maybe someone will make use of the info at at IB..... and save the next customer a good 1.5 hrs of headache.