There are many reasons, one of the more common however has proven to be related to php version. If your code uses mysql_connect etc rather then mysqli or PDO, you would need to downgrade to php version 5.6, all accounts are set on php version 7.2 on the new servers as that is its default option.
Most modern scripts that are kept up to date would use the newer versions though as they are more secure.
Under "System Info & Files" you can find the error log, that could hold some clues as to what is causing that error.