Problem:
Solarium 5.0.2 is too old and has known issues. In order to get composer to upgrade solarium to a recent version a core patch or a workaround is required. Have a look at this Core issue and this Search API Solr issue.
Lösung:
composer require symfony/event-dispatcher:"4.3.4 as 3.4.99" && composer update
Comments
SolrCore Initialization…
SolrCore Initialization Failures
site5: org.apache.solr.common.SolrException:org.apache.solr.common.SolrException: Could not load conf for core itlntk: Can't load schema /var/solr/data/site5/conf/schema.xml: Plugin init failure for [schema.xml] fieldType "collated_en": Error loading class 'solr.ICUCollationField'
Please check your logs for more information
composer require symfony…
composer require symfony/event-dispatcher:"4.3.4 as 3.4.99"
composer update