aprocter on "404/500 errors on brand new install"

I've been having lots of issues with WP lately, so I thought I'd install a brand new installation of 4.3.

Using my current webhost (arvixe), I used the automated script to install WP 4.3. This is a brand new install, letting it create the folder - which just happens to be a domain I own that was not doing anything. It creates the folder, everything, just fine. I can go to http://www.mprocter.com and everything is there. I can log in using my admin account. Everything looks good. Until I start trying to go to some of the more "exotic" features, like Theme, or Plugins, at which point I get the dreaded

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, webmaster@mprocter.com and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

The log file shows the following:

[Wed Aug 26 22:33:30 2015] [error] [client w.x.y.z] File does not exist: /home/xxx/public_html/mprocter.com/500.shtml, referer: http://mprocter.com/wp-admin/themes.php
[Wed Aug 26 22:29:24 2015] [error] [client w.x.y.z] File does not exist: /home/xxx/public_html/mprocter.com/500.shtml
[Wed Aug 26 22:28:18 2015] [error] [client w.x.y.z] File does not exist: /home/xxx/public_html/mprocter.com/500.shtml, referer: http://mprocter.com/wp-admin/themes.php
[Wed Aug 26 22:28:17 2015] [error] [client w.x.y.z] File does not exist: /home/xxx/public_html/mprocter.com/500.shtml, referer: http://mprocter.com/wp-admin/themes.php
[Wed Aug 26 22:27:52 2015] [error] [client w.x.y.z] File does not exist: /home/xxx/public_html/mprocter.com/500.shtml, referer: http://mprocter.com/wp-admin/themes.php
[Wed Aug 26 22:27:40 2015] [error] [client w.x.y.z] File does not exist: /home/xxx/public_html/mprocter.com/500.shtml

I've tried renaming .htaccess, and Themes are now working, but plugins aren't. What am I missing here? Has the softaculous script failed to set up something correctly? I've never had an issue in the past, but this is a clean install, clean database, clear directory, and it's still failing.

Adrian