Problem and Symptoms
All of a sudden, starting January 10, 2022, the spell checking service, after clicking on the "spellcheck" under the ABC button in CKEditor 4, stopped working despite the fact that previously it worked properly. As a result you see incessant loading spinner.
Root cause
As was announced a year ago, effective January 10, 2022, we disabled the Free Cloud version of the WebSpellChecker (WSC) plugin for CKEditor 4. This is the end-of-life for this plugin. Users that have added WebSpellChecker (WSC) plugin to the CKEditor config.js file using the extraPlugins parameter or still using CKEditor lower then 4.16.0 may face an issue with the loading spinner.
Solution
To solve the issue, remove the WSC Dialog plugin from extraPlugins parameter in the CKEditor config (if any) or disable it using the removePlugins parameter:
CKEDITOR.config.removePlugins = 'wsc';
Alternatively, instead of the WSC Dialog plugin, you can use the Free SCAYT service.
