I tried everything I could think of and many, many forums discussing this issue with no help at all until I found this post. I followed the directions step-by-step and it worked great! Basically jUpgrade 2.5.0 and jUpgrade 2.5.1 do not support Joomla 2.5.5 or later. Hopefully they will fix their mistake.
jUpgrade Issue with MySQLiWritten by Chet King Ventures
Two of the latest websites I have been working to upgrade from Joomla 1.5 to Joomla 2.5 with jUpgrade have gotten stuck at the "Migrating undefined" step.
Published in Business Blog