Support wise we offer full technical support via ticket support, this does not cover customization or 3rd party code/addons etc however.
In terms of importing your data, you would be best contacting various providers listed here: https://xenforo.com/community/forums/third-party-services-offers.42/
It would appear if one of the first threads is more than 500 posts this causes it to restart.
A simple fix for now is to increase const MAX_POSTS = 500;
/src/addons/XFI/Import/Importer on line 22
to a higher number.
I see you haven't linked your forum account to your license. You can do that in the Forum Users section of the Customer Area here: http://xenforo.com/customers
Then we will be able to provide forum support.