Check if your suitecrm.log file is being created and/or updated. If not then HTTP Error 500 could be an IIS Permissions issue.
Settings (which maybe could be made less permissive, but work):
All of these are set for the (say) c:\inetpub\wwwroot\suitecrm8 folder:
- IIS AppPool\<your app pool name the site uses> should have full control
- Give IUSR user full control
- Give IIS_IUSRS full control.
- Restart site.
…jack