I cannot index any content from my Drupal CMS to my Opensolr index

Drupal stops indexing, and the Error Log of your index shows an error like this one:

org.apache.solr.common.SolrException: java.io.FileNotFoundException:
/usr/share/solr/example/multicore/INDEX_NAME/data/tlog/tlog.0000000000000002357
(No such file or directory)

The transaction log of the index (the tlog, where Solr records recent updates) refers to a file that no longer exists, so Solr refuses new updates.

01 · Fix it

Reload the index

In the Control Panel of the index: Tools › Utilities › Reload. Reload.

If the error stays or comes back, remove the update log

Click Configuration, open solrconfig.xml in the editor, delete the whole updateLog block shown below and save. Saving reloads the index by itself.

On a dedicated server

Tools › Restart Solr Service restarts Solr: Restart Solr Service.

The block looks like this; the lines inside it can differ:

<updateLog>
  <str name="dir">${solr.ulog.dir:}</str>
  <int name="numVersionBuckets">${solr.ulog.numVersionBuckets:65536}</int>
</updateLog>

Without an update log, Solr cannot replay updates that were not committed when it stopped; Drupal can send those items again. The config.zip of Search API Solr contains the block, so remove it again after each new upload.

02 · Then

Index again from Drupal: Index now on your Search API index, or Ingest All Now with the Opensolr Search module.