Saves aren't generally transferable between major versions and 3.0 was a big one.
The problem is likely that one of the vassals on the winning side of the crusade owns a wasteland.
Between now and the version you started in, a province has been changed from being a normal province to being a wasteland and due to this, there is an owner of that wasteland in your save data. Wastelands should never be owned by someone and therefore this causes a crash.
Despite this save game transfer not being supported, I have made a fix for this particular crash for the upcoming patch.
But it's unfortunately quite likely that more issues will appear even if you make it past this crash as the save data is in an odd state due to the transfer.