Anomalous log-in dashboard screen

I just installed Perch on its own (without importing an already developed site) on a live server in a dirt cheap shared hosting environment. I know I'm violating best practices, but I thought this behavior I encountered was weird enough to warrant your attention.

When I try to log in with the correct password, I get a weird hybrid of the dashboard and the log-in screen. The toggle sidebar pane is visible and functional, but there's a partially obscured log in field, along with the "Log in" button and the "Reset password" link.

Trying to type something into that broken field and clicking "Log in" can lead back to a standard log-in screen, with the password field highlighted in red as incorrect.

I tried logging in with an incorrect password, and these PHP warnings were output above the resulting standard log-in screen displaying the standard error message "Sorry, those details are incorrect ...":

Warning: escapeshellcmd() has been disabled for security reasons in /home/a5725/public_html/perch/core/lib/PerchAuthenticatedUser.class.php on line 131

Warning: Cannot modify header information - headers already sent by (output started at /home/a5725/public_html/perch/core/lib/PerchAuthenticatedUser.class.php:131) in /home/a5725/public_html/perch/core/inc/auth.php on line 29

Warning: Cannot modify header information - headers already sent by (output started at /home/a5725/public_html/perch/core/lib/PerchAuthenticatedUser.class.php:131) in /home/a5725/public_html/perch/core/inc/auth.php on line 50

Warning: Cannot modify header information - headers already sent by (output started at /home/a5725/public_html/perch/core/lib/PerchAuthenticatedUser.class.php:131) in /home/a5725/public_html/perch/core/lib/PerchUtil.class.php on line 1211

Warning: Cannot modify header information - headers already sent by (output started at /home/a5725/public_html/perch/core/lib/PerchAuthenticatedUser.class.php:131) in /home/a5725/public_html/perch/core/lib/PerchUtil.class.php on line 1212

Warning: Cannot modify header information - headers already sent by (output started at /home/a5725/public_html/perch/core/lib/PerchAuthenticatedUser.class.php:131) in /home/a5725/public_html/perch/core/lib/PerchUtil.class.php on line 1213

Warning: Cannot modify header information - headers already sent by (output started at /home/a5725/public_html/perch/core/lib/PerchAuthenticatedUser.class.php:131) in /home/a5725/public_html/perch/core/lib/PerchUtil.class.php on line 1219

Warning: Cannot modify header information - headers already sent by (output started at /home/a5725/public_html/perch/core/lib/PerchAuthenticatedUser.class.php:131) in /home/a5725/public_html/perch/core/inc/top.php on line 17

I don't blame the CMS for my cheap hosting choking on it. I believed the hosting provider meets the minimum server requirements, according to its advertising. However, I don't understand some of the fine details of the requirements. It's running PHP 5.6, but I don't know what PDO is, or how to tell if mysqli or JSON are implemented.

(I never tried installing the compatibility test. Maybe I should try that, if I have time. But I probably won't. I'll just delete the Perch install out of this cheap host and try it in another cheap host I was going to try, and I'll move my WordPress website over to this cheap host because they've got a WordPress installation guide.......)