Without a report, it’s impossible for us to know for sure. But usually this happens because people remove some of the units that move in events. If you try to move a unit that no longer exists, the game will freeze unless you apply the fix.
To make a report, go to File → Create Report Issue. Then post the .7z file in FE_Builder_GBA -- If you have any questions, attach report7z or in the FEBuilder channel in FEU’s Discord server.