First error: ConvertLead failed. First exception on row 1; first error: INVALID_STATUS, invalid convertedStatus: Qualified: [Status]

after another API version update there is a new field representing closing status. instead of MasterLabel you should use ApiName, they may differ.

so the code snippet inside mergLeadWithExistingAccount() method should look like:

LeadStatus convertStatus = [SELECT Id, ApiName FROM LeadStatus WHERE IsConverted=true LIMIT 1];
lc.setConvertedStatus(convertStatus.ApiName);