I too have raised a ticket on this it seems that the migration has failed part way through and not errored!
When I have browsed the database I discover that the event IDs in the User table are changed and the event ids in the group_member table are not. I have tried resynching using SQL using the SLABID and this works apart from a small number of records that used the SLABID for another event.
I am using Jevents for the event calendar and so I need to ensure that the linkage works up and down to the Jevents system
I hope that my ticket gets a look at soon, I have received the message that I did not follow all the steps in the readme properly, however I have now done this on my test system 5 times and deleted ALL of the 275 tables before restoring to 269 and I keep getting the same results. I have reviewed the videos and walked through the readme process umpteen times so I am pretty sure I have done it right, I have opened every configuration item and all fields and resaved them too.
It would be useful to know what the table relationship should be as I suspect that with the option not to use Jevents that DTRegister is now using intermediate tables to relate the eventid but without a bit of guidance it is a long slog through the tables with a browser. Ideally I would like DTH to take a look and confirm the situation but they seem to be hyper busy right now..