I had this problem too. Do you have phpmyadmin open? I noticed that once I closed out of that, it worked. Not sure if there is some sort of login-locking mechanism or if it is a fluke...but I've had this happen a couple times and closing phpmyadmin worked every time.
I've definitely got 1.51 then. And it wouldn't work until I removed those spaces.
interestingly, with the built in 3.7/3.8 importer each of the fields was pre-filled with a $config line.
Before I deleted the spaces, those fields were blank in your vB4 importer. once I deleted the spaces, the next time I tried to import using the vB4 importer all the fields were filled with $config, just like the 3.7 one.
I know this could me outside the scope of the VB4 importer but i was wondering what is the best way to do a new import (since the first that was done was made to test everything on a new forum) without the need to reinstall everitying and keep all the setings in the XF control panel ???
Deleting all forums and users should be ok ? or there is a smarter approach to this ??? Or maybe these should be a option to sync the import somehow ???
What's the best way to start a fresh import after running a "test" sequence - while keeping the current XenForo general settings?
Re-run the importer? Will it replace or duplicate data? Or maybe there's a way to drop only the imported data, so it can be imported again from vBulletin?
The only way to get a fresh start is to restore a backup from before the import or to do a reinstall (you will have to reconfigure things obviously). A full import puts a lot of data into the system, and removing it all manually would be far more time consuming than doing a reinstall and reconfig.