Problem found, a2dismod php5
followed by systemctl restart apache2
removed the php5 module that was apparently incorrectly (in addition to php7) activated during the update.
No idea why php5 is still installed here:
130 root@XXXXX /etc/apache2 # dpkg-query -l | grep php :(
ii libapache2-mod-php5 5.6.39+dfsg-0+deb8u1 amd64 server-side, HTML-embedded scripting language (Apache 2 module)
ii libapache2-mod-php7.0 7.0.33-0+deb9u7 amd64 server-side, HTML-embedded scripting language (Apache 2 module)
ii php 1:7.0+49 all server-side, HTML-embedded scripting language (default)
ii php-cgi 1:7.0+49 all server-side, HTML-embedded scripting language (CGI binary) (default)
ii php-common 1:49 all Common files for PHP packages
ii php-date 1.4.7-3 all Generic date/time handling class for PEAR
ii php-enchant 1:7.0+49 all Enchant module for PHP [default]
ii php-gettext 1.0.12-0.1 all transitional dummy package for php-php-gettext
ii php-kopano-smime 1.0.00+4.1 amd64 PHP Kopano SMIME Extension extends the php-openssl functions.
ii php-krb5 1.1.2-4A~4.3.0.201802091342 amd64 krb5 module for PHP
ii php-ldap 1:7.0+49 all LDAP module for PHP [default]
ii php-mbstring 1:7.0+49 all MBSTRING module for PHP [default]
ii php-mcrypt 1:7.0+49 all libmcrypt module for PHP [default]
ii php-memcache 3.0.9~20160311.4991c2f-5 amd64 memcache extension module for PHP
ii php-pear 1:1.10.1+submodules+notgz-9+deb9u1 all PEAR Base System
ii php-php-gettext 1.0.12-0.1 all read gettext MO files directly, without requiring anything other than PHP
ii php-smbclient 0.9.0-1+48.1 amd64 libsmbclient-php is a PHP extension that uses Samba's libsmbclient
ii php-soap 1:7.0+49 all SOAP module for PHP [default]
ii php-xml 1:7.0+49 all DOM, SimpleXML, WDDX, XML, and XSL module for PHP [default]
ii php-zip 1:7.0+49 all Zip module for PHP [default]
ii php5 5.6.39+dfsg-0+deb8u1 all server-side, HTML-embedded scripting language (metapackage)
ii php5-cgi 5.6.39+dfsg-0+deb8u1 amd64 server-side, HTML-embedded scripting language (CGI binary)
ii php5-cli 5.6.39+dfsg-0+deb8u1 amd64 command-line interpreter for the php5 scripting language
ii php5-common 5.6.39+dfsg-0+deb8u1 amd64 Common files for packages built from the php5 source
ii php5-curl 5.6.39+dfsg-0+deb8u1 amd64 CURL module for php5
ii php5-enchant 5.6.39+dfsg-0+deb8u1 amd64 Enchant module for php5
ii php5-gd 5.6.39+dfsg-0+deb8u1 amd64 GD module for php5
rc php5-gmp 5.6.39+dfsg-0+deb8u1 amd64 GMP module for php5
ii php5-intl 5.6.39+dfsg-0+deb8u1 amd64 internationalisation module for php5
ii php5-json 1.3.6-1 amd64 JSON module for php5
rc php5-ldap 5.6.39+dfsg-0+deb8u1 amd64 LDAP module for php5
rc php5-mcrypt 5.6.39+dfsg-0+deb8u1 amd64 MCrypt module for php5
rc php5-memcache 3.0.8-5 amd64 memcache extension module for PHP5
rc php5-mysql 5.6.30+dfsg-0+deb8u1 amd64 MySQL module for php5
rc php5-pgsql 5.4.45-0.238.201704191554 amd64 PostgreSQL module for php5
ii php5-readline 5.6.39+dfsg-0+deb8u1 amd64 Readline module for php5
ii php5-sqlite 5.6.39+dfsg-0+deb8u1 amd64 SQLite module for php5
ii php5-tidy 5.6.39+dfsg-0+deb8u1 amd64 tidy module for php5
rc php5-zendframework2 2.3.5-3.1 amd64 Zend Framework 2
ii php7-mapi 8.7.9.0-0+16.1 amd64 PHP bindings for MAPI
ii php7.0 7.0.33-0+deb9u7 all server-side, HTML-embedded scripting language (metapackage)
ii php7.0-bcmath 7.0.33-0+deb9u7 amd64 Bcmath module for PHP
ii php7.0-cgi 7.0.33-0+deb9u7 amd64 server-side, HTML-embedded scripting language (CGI binary)
ii php7.0-cli 7.0.33-0+deb9u7 amd64 command-line interpreter for the PHP scripting language
ii php7.0-common 7.0.33-0+deb9u7 amd64 documentation, examples and common module for PHP
ii php7.0-curl 7.0.33-0+deb9u7 amd64 CURL module for PHP
ii php7.0-enchant 7.0.33-0+deb9u7 amd64 Enchant module for PHP
ii php7.0-json 7.0.33-0+deb9u7 amd64 JSON module for PHP
ii php7.0-ldap 7.0.33-0+deb9u7 amd64 LDAP module for PHP
ii php7.0-mbstring 7.0.33-0+deb9u7 amd64 MBSTRING module for PHP
ii php7.0-mcrypt 7.0.33-0+deb9u7 amd64 libmcrypt module for PHP
ii php7.0-opcache 7.0.33-0+deb9u7 amd64 Zend OpCache module for PHP
ii php7.0-readline 7.0.33-0+deb9u7 amd64 readline module for PHP
ii php7.0-soap 7.0.33-0+deb9u7 amd64 SOAP module for PHP
ii php7.0-xml 7.0.33-0+deb9u7 amd64 DOM, SimpleXML, WDDX, XML, and XSL module for PHP
ii php7.0-zip 7.0.33-0+deb9u7 amd64 Zip module for PHP
ii simplesamlphp 1.16.3-1+deb10u1A~4.4.0.201911111614 all Authentication and federation application supporting several protocols
ii univention-saml 6.0.2-25A~4.4.0.202003121622 all Integrates simpleSAMLphp Identity Provider into UCS
ii univention-saml-schema 6.0.2-25A~4.4.0.202003121622 all UCS simpleSAMLphp ldap integration
Something must be wrong here, because with all the recent minor & patchlevel/point updates the problem did not exist.
Robert