Most likely your new host, something is missing. Try before you deploy the old version to put a clean from scratch. When you install installer will be to write him wrong.
Nigel_Streich answered on July 2nd 19 at 17:43
0. View the logs. Perhaps there is /core/cache/logs/
1. Remove all contents of the folder /core/cache/
2. Check that specified the correct dB connection in core/config/config.inc.php
3. Check that correctly pointed out the path in files:
Khalid59 answered on July 2nd 19 at 17:45
500 error is a General error to understand the fix, you need to see the logs of PHP or the web server.
Eulalia22 answered on July 2nd 19 at 17:47
Go into the Apache logs
melyna_Lindgr answered on July 2nd 19 at 17:49
Just today decided the same issue from the client. It turned out that his hosting has accumulated a lot of letters (Recent, the boxes were not limited in size and rarely checked) and all the free lot was full because of this admin would not open. Cleaned and admin work. Check if you have the same thing.