500 Internal Server Error

I have upgraded the system to Ubuntu 18.04.1 LTS and 2 sites received the error. Others appear to be working correctly. I have read that this might be related to htaccess.txt so I have posted from one of the sites below. This particular site at first got the error and now just shows a blank page.

##
# @package Joomla
# @copyright Copyright (C) 2005 - 2014 Open Source Matters. All rights reserved.
# @license GNU General Public License version 2 or later; see LICENSE.txt
##

##
# READ THIS COMPLETELY IF YOU CHOOSE TO USE THIS FILE!
#
# The line just below this section: 'Options +FollowSymLinks' may cause problems
# with some server configurations. It is required for use of mod_rewrite, but may already
# be set by your server administrator in a way that dissallows changing it in
# your .htaccess file. If using it causes your server to error out, comment it out (add # to
# beginning of line), reload your site in your browser and test your sef url's. If they work,
# it has been set by your server administrator and you do not need it set here.
##

## Can be commented out if causes errors, see notes above.
Options +FollowSymLinks

## Mod_rewrite in use.

RewriteEngine On

## Begin - Rewrite rules to block out some common exploits.
# If you experience problems on your site block out the operations listed below
# This attempts to block the most common type of exploit `attempts` to Joomla!
#
# Block out any script trying to base64_encode data within the URL.
RewriteCond %{QUERY_STRING} base64_encode[^(]*\([^)]*\) [OR]
# Block out any script that includes a <script> tag in URL.
RewriteCond %{QUERY_STRING} (<|%3C)([^s]*s)+cript.*(>|%3E) [NC,OR]
# Block out any script trying to set a PHP GLOBALS variable via URL.
RewriteCond %{QUERY_STRING} GLOBALS(=|\[|\%[0-9A-Z]{0,2}) [OR]
# Block out any script trying to modify a _REQUEST variable via URL.
RewriteCond %{QUERY_STRING} _REQUEST(=|\[|\%[0-9A-Z]{0,2})
# Return 403 Forbidden header and show the content of the root homepage
RewriteRule .* index.php [F]
#
## End - Rewrite rules to block out some common exploits.

## Begin - Custom redirects
#
# If you need to redirect some pages, or set a canonical non-www to
# www redirect (or vice versa), place that code here. Ensure those
# redirects use the correct RewriteRule syntax and the [R=301,L] flags.
#
## End - Custom redirects

##
# Uncomment following line if your webserver's URL
# is not directly related to physical file paths.
# Update Your Joomla! Directory (just / for root).
##

# RewriteBase /

## Begin - Joomla! core SEF Section.
#
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
#
# If the requested path and file is not /index.php and the request
# has not already been internally rewritten to the index.php script
RewriteCond %{REQUEST_URI} !^/index\.php
# and the request is for something within the component folder,
# or for the site root, or for an extensionless URL, or the
# requested URL ends with one of the listed extensions
RewriteCond %{REQUEST_URI} /component/|(/[^.]*|\.(php|html?|feed|pdf|vcf|raw))$ [NC]
# and the requested path and file doesn't directly match a physical file
RewriteCond %{REQUEST_FILENAME} !-f
# and the requested path and file doesn't directly match a physical folder
RewriteCond %{REQUEST_FILENAME} !-d
# internally rewrite the request to the index.php script
RewriteRule .* index.php [L]
#
#
php_value post_max_size 131072kb
php_value upload_max_filesize 131072kb
#
## End - Joomla! core SEF Section.

 

Here's what I saw in error log for the site:
[Mon Dec 17 11:24:23.916884 2018] [fcgid:warn] [pid 13040] [client 46.229.168.153:44746] mod_fcgid: stderr: PHP Fatal error: Cannot use Joomla\\String\\String as String because 'String' is a special class name in /var/clients/client0/web90/web/libraries/vendor/joomla/registry/src/Format/Json.php on line 12
[Mon Dec 17 11:24:23.916906 2018] [fcgid:warn] [pid 13040] [client 46.229.168.153:44746] mod_fcgid: stderr: PHP Warning: session_write_close(): Failed to write session data (user). Please verify that the current setting of session.save_path is correct (/var/clients/client0/web90/tmp) in Unknown on line 0

 

I don't know that it's using PHP 7.2. The other sites actually are complaining that they are still on 7.0. I think that's the current default.

I reset the site that was showing a blank page to PHP 5.4 and now it's again showing the Internal Error message. That may be where it was before. Here's what the site's error log says now:
[Mon Dec 17 12:24:16.514855 2018] [fcgid:warn] [pid 25694] (104)Connection reset by peer: [client 46.229.168.146:43750] mod_fcgid: error reading data from FastCGI server
[Mon Dec 17 12:24:16.514888 2018] [core:error] [pid 25694] [client 46.229.168.146:43750] End of script output before headers: index.php