Drupal reports the Solr server as unreachable, or indexing stops with an error. Go through these checks in order: the first three explain most cases.
01 · Checks
Solr path and Solr core
Solr path is a single slash, /. Solr core is the name of your index. The solr/ path on the Info tab is added by Search API Solr itself. The whole form: Drupal Configuration Steps.
Username and password
Use the Basic Auth connector, or Opensolr with Basic Auth from Search API Opensolr, with the username and password of the index from its Security tab. Letters and digits only. New values work after about 10 seconds, and a new API key does not change the password of the indexes you already have: Default HTTP Auth Credentials.
Host and port
The Host from the Info tab, without https://, with port 443 and the https protocol. Indexes in China use port 80 and http.
IP restriction
If you added IP rules in the Security tab, the IP address of your web server must be on the list: IP restriction.
403 on every request
An index over its disk space or bandwidth limit answers 403 to every request, searches included, until usage is back under the limit. Your Dashboard shows both: Disk space, Bandwidth.
Configuration files
Upload the config.zip that Search API Solr builds for your index (Get config.zip on the server page) on the Configuration tab of the index. If the upload reports an error: Can not update/upload configuration files.
02 · A 404 on the index address is normal
Opening https://HOST/solr/my_index/ in a browser answers 404 NOT FOUND: the index answers only on its handlers, such as /select. Test one of them instead:
curl -u opensolr:YOUR_API_KEY "https://HOST/solr/my_index/select?q=*:*&wt=json"
A list of results means the connection works.
03 · Video
Common Drupal and Opensolr connection issues, step by step.