If you are on JIRA 7.x and connector version 6.0.11 and onwards, you may enable a DEBUG logging to help us identify what has exactly broken, the next time this problem happens:
Go to Cog Icon >System.
Under Troubleshooting and Support in the sidebar, click Logging and profiling.
Under Default Loggers section, click Configure logging level for another package.
Enter package name as net.customware.plugins.connector.salesforce.soap, with logging level as DEBUG.
Click Add.
If the problem happens again, please gather your atlassian-jira.log files and send to us at support@servicerocket.com.
There are two likely causes for this:
Your Salesforce.com account credentials have expired
The license for the Connector for Salesforce.com and JIRA has expired
The following are suggestions on how to resolve these issues:
Your Salesforce.com account credentials have expired
Go to Cog Icon >Add-ons.
Under Cloud Connectors in the sidebar, click Connection.
Look for your Salesforce.com Connection, then click Test.
Verify if the connection is successful. If it isn't, expand the error details. There is a high chance you will get an error similar to "Invalid username/password".
If this is the case, reset your Salesforce.com account password and update the Salesforce.com authentication details in your JIRA configuration.
The license for the Connector for Salesforce.com and JIRA has expired
Go to Cog Icon > Add-ons.
Under Cloud Connectors, click Licensing and verify that your Connector for Salesforce.com and JIRA license is still valid.
Alternatively, you will also see the "License is expired" error message while performing a test connection.
The solution is to renew the existing license and re-upload a new one. Or you could also obtain an evaluation license as a temporary workaround while you wait for approval to renew your license.