To eliminate the problem with the customizer may need to increase a particular Apache service limit. That is because you have a lot of plugins activated and that “eats” your resources. The cause of the problem is that these plugins are reaching a certain limit applied to the Apache service. In order to resolve the issue please add the SubstitueMaxLineLength 10M parameter to your .htaccess file on both production and dev websites.
The WordPress default part needs to look like that in the .htaccess file:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
<IfModule mod_substitute.c>
SubstituteMaxLineLength 10M
</IfModule>
# END WordPress
By the way, it is a known issue that may happen on some shared and virtual hostings especially on SiteGround and GoDaddy.