i had some problems with:,

"Fatal error
Transaction error
Unable to parse AJAX response."

while installing in joomla 3.2 sandbox,
some more info was in joomla 2.5 sandbox:

"Specified key was too long; max key length is 1000 bytes"

Note: UNIQUE is also a KEY so adding to much UNIQUE columns in one table will trigger that error,

SOLUTION: reduce the size of unique columns (default for string is varchar 255 set to 15 for ex. in column options i j-cook)

This Issue is finaly fixed.

When you define a unique statement in the field properties, the builder show you how many bytes are left.
www.j-cook.pro/index.php/docs/versions/199-2-7