Complaints about credit reporting errors made up almost one-third of disputes taken to the Credit & Investments Ombudsman over the past year, overtaking complaints about financial hardship applications as the ombudsman service's biggest issue.Twenty-nine per cent of complaints to CIO in the 2015/16 financial year were about credit reporting - up from 24 per cent last year. Twenty-one per cent of complaints related to financial hardship.The financial services providers that were subject to most complaints were debt purchasers and collectors, accounting for 42 per cent of all complaints (up from 40.7 per cent last year).Almost 40 per cent of the complaints about debt purchasers were about incorrect listings on credit reports.CIO pointed out that the complaints about debt purchasers might relate to the conduct of the debt purchaser or the original credit provider. Once the debt has been sold CIO requires the purchaser to deal with complaints about the original credit provider's conduct.The ombudsman identified several systemic issues and instances of serious misconduct in its investigation of credit reporting complaints. These included listing incorrect default amounts, failing to provide notice of the default amount and inappropriate listings.Among debt purchasers and collectors, Credit Corp Recoveries attracted 354 complaints (up from 349 last year), Baycorp Collections 270, Lion Finance 182, Pioneer Credit 148 and ACM Group 125.Overall, CIO received 4760 complaints in 2015/16 - a 1.8 per cent reduction compared with the previous year but 5.5 per cent more than the number received in 2013/14.CIO membership increased 12.2 per cent to 22,973.The great majority of consumers handled their complaints themselves but 6.9 per cent used a credit repair or debt negotiation service, 2.9 per cent used a financial counselor, 2.1 per cent used a community legal centre and 1.5 per cent used a solicitor.