We updated search_api_solr to version 4.3.1. When upgrading the module to 4.3.x from 4.2.x and earlier you still can read an existing Solr index and perform searches. But if you write (index) data you'll run into errors.
To fix this issue you need to recreate the solr core with a new config. If you don't know how to recreate the solr core you can read about it here:
https://git.drupalcode.org/project/search_api_solr/-/blob/4.x/README.md?...
If you are using Solr 8:
You can use the config template on https://git.drupalcode.org/project/search_api_solr/-/tree/4.x/jump-start.... This is the default config to get it up and running. After running the new config you can download a more specific config for your environment using the drush command
drush solr-gsc [solr_server_machine_name] config.zip 8.6
if you are using Solr 9
You can use the config template on https://git.drupalcode.org/project/search_api_solr/-/tree/4.x/jump-start.... This is the default config to get it up and running. After running the new config you can download a more specific config for your environment using the drush command
drush solr-gsc [solr_server_machine_name] config.zip 9.2
