If still receiving the same error, try disabling and re-enabling the Watchlist.
Make note of any other console issues that may be symptoms of a larger issue.
Check the /var/log/cb/job-runner/jog-runner.log for the Watchlist name, to see if other errors are present at that time.
Check the /var/log/cb/solr*/debug.log and startup.log to look out for errors at the time of reproducing this issue.
If none of the above helped, upload cbdiags to alliance.
Additional Information
If the issue is happening on a specific Watchlist, disable the current Watchlist and create a new Watchlist with the same query as well as notification settings.
If Solr cores are not optimizing, the server may not be meeting the Operating Environment Requirements standards for the amount of data coming through, slowing down any requests to the database.