I can no longer connect to JIRA by providing my username and password under Administration -> Ticketing System. What is wrong?

Since 6/5/2019 basic authentication with email/password was disabled for REST API usage of Atlassian JIRA. You should generate an API token instead and use it in the password field by following the instructions below: https://confluence.atlassian.com/cloud/api-tokens-938839638.html.