An http error 500 can be due to many things, but one of the more common is mismatched php version.
All accounts on the new servers are set to php version 7.2 by default, this will cause a problem if your code for example rely on the old and now removed mysql extension, it is replaced with more secure options: mysqli and PDO.
You have the option to switch to php version 5.6 which allows for use of the older mysql extension, though the recommendation would of course be to make sure your code is updated to use the more secure versions.
There may also be other causes, the error log may hold clues and can be found under "System Info & Files"