com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Can't DROP 'fk_ao_7b9828_XXXXXXXXX'; check that column/key exists

We don't know how this error occurs. It is not specific to WorklogPRO and can happen to any add-on as discussed here. Solving this error requires you to run DDL statements in your database. What you need to do is: 

  1. Remove WorklogPRO add-on
  2. Stopped Jira
  3. Export data from all tables with AO_7B9828 prefix
  4. Drop all of the tables with AO_7B9828 prefix
  5. Started JIRA
  6. Install WorklogPRO add-on
  7. Re-import the data exported in step 3