Steps to reproduce the issue

--- Live Hosting ----
On Hosting provider (SiteGround) I go to the PHP selector in cPanel and choose the version PHP 7.2.3 for the specific folder my site is in. Save the setting, then go to www.murraywebs.biz/dev/administrator
Login screen is shown normally.
Login works OK.
Once logged in, it loads an unstyled Isis admin screen, with not all options and links available (no main menu etc).

Same issue on my local PC:
--Localhost--
Using WAMPServer 3.1.1.
Switch PHP to 7.2.x
Restart server etc
Isis template loads with no styling (just black text on white screen)

Expected result

Should load Isis template normally under PHP 7.2.x.

Actual result

No styling. The login screen is normal, but on logging on, the screen turns tot he black text on white screen.

The same issue occurs on my local PC (localhost running WAMPServer 3.1.1). I have PHP 7.15, 7.2.0, 7.2.2 on this system and same issue occurs when any of the PHP 7.2.x versions are selected, so it seems not just my live host.

System information (as much as possible)

This was occurring in Joomla 3.8.5 and 3.8.6

Site Ground Hosting (shared hosting)
PHP 7.2.3
Have a choice of PHP versions:

PHP 5.5.38
PHP 5.6.34 - System Default
PHP 7.0.28
PHP 7.1.15
PHP 7.2.3

Using LetsEncrypt SSL.

Additional comments

When I switch the PHP version back go 7.1.5, and refresh the screen, the Isis administrator template loads normally, so I have concluded it is a problem with PHP 7.2.x