Hey,
search_api_solr is incompatible with search_api since yesterday's release 1.35.
The following error occurs right after updating to 1.35 of search_api
Declaration of Drupal\search_api_solr\Plugin\search_api\backend\SearchApiSolrBackend::__sleep() must be compatible with Drupal\search_api\Backend\BackendPluginBase::__sleep(): array in C:\xampp_htdocs\drupolis_dev\docroot\modules\contrib\search_api_solr\src\Plugin\search_api\backend\SearchApiSolrBackend.php on line 5077
Closed: duplicate
4.3
Code
I am sorry, it is already fixed by https://git.drupalcode.org/project/search_api_solr/-/commit/ff2c18354a24...
see 🐛 Fatal error Declaration of Drupal\search_api_solr\Plugin\search_api\backend\SearchApiSolrBackend::__sleep() Fixed