You probably should run the system test.
The culprit may be that your server software is beyond what those older versions supported to upgrade, you may need to downgrade your php version to php5.3 or php5.4 if you can till ready for UBB7.74 which will run on php5.4 to php7

He is correct about one item , in the importer to goto ubb7.X has a include that requires
require ("../libs/mysql.inc.php");
require ("../libs/ubbthreads.inc.php");

They all do not exist in the archive till ubb 7.3.0

Blue Man Group
There is no such thing as stupid questions. Just stupid answers