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problem with Apache/php after upgrading to 18.04

I just updated The OS from ubuntu 16.04 LTS to ubuntu 18.04 LTS.
The login page loads fine, but when the main page loads it
displays the php script that is loaded via AJAX as text, ie it is
not interpreted.
Then. after about 1 second, the page exits and goes back to the
login page.

A login failure redirects to the login page without displaying
any of the main page. So this is not a login failure.

Everything was fine under ubuntu 16.04 LTS.

Any thought as to what might be happening and/or any thoughts on
debugging it.

Here is the Apache2 error log:
...
[Wed Jan 23 09:50:18.803176 2019] [core:notice] [pid 1603]
AH00094: Command line :
'/usr/sbin/apache2'
[Wed Jan 23 14:53:09.818285 2019] [mpm_prefork:notice] [pid 1603]
AH00169: caugh t SIGTERM,
shutting down
[Wed Jan 23 14:53:09.949489 2019] [mpm_prefork:notice] [pid 2933]
AH00163: Apach e/2.4.29
(Ubuntu) configured -- resuming normal operations
[Wed Jan 23 14:53:09.949550 2019] [core:notice] [pid 2933]
AH00094: Command line :
'/usr/sbin/apache2'
[Wed Jan 23 15:02:24.216059 2019] [mpm_prefork:notice] [pid 2933]
AH00169: caugh t SIGTERM,
shutting down
[Wed Jan 23 15:03:17.252870 2019] [mpm_prefork:notice] [pid 1426]
AH00163: Apach e/2.4.29
(Ubuntu) configured -- resuming normal operations

-bill