"You are using a fallback implementation of the intl extension. Installing the native one is highly recommended instead. at \/var\/www\/clients\/client0\/web4\/web\/3rdparty\/patchwork\/utf8\/src\/Patchwork\/Utf8\/Bootup\/intl.php#18"

ℹ️ Support
10

Just to show the precise problem and solution:

My PHP 7.3 comes from deb.sury.org, and they have a php7.3-intl module

root@blackbox:~# apt-cache policy php7.3-intl
php7.3-intl:
  Installed: 7.3.7-1+0~20190710.40+debian8~1.gbp032aec
  Candidate: 7.3.7-1+0~20190710.40+debian8~1.gbp032aec
  Version table:
 *** 7.3.7-1+0~20190710.40+debian8~1.gbp032aec 0
        500 https://packages.sury.org/php/ jessie/main amd64 Packages
        100 /var/lib/dpkg/status

There’s a second package without a version (7.3) in its name, and you need it too:

root@blackbox:~# apt-cache policy php-intl
php-intl:
  Installed: (none)
  Candidate: 2:7.3+69+0~20190303094641.15+jessie~1.gbp0f7465
  Version table:
     2:7.3+69+0~20190303094641.15+jessie~1.gbp0f7465 0
        500 https://packages.sury.org/php/ jessie/main amd64 Packages

the gist is: If you install PHP from a not-Debian source then all the modules have to come from this source too.