Software quality assurance to comply with user requirements enables software development companies to be competitive. Maintaining a high quality level requires continuous monitoring and development. If there are quality problems, the company’s reputation is suffering and its costs increase because of investing in time and eliminating the consequences of the problems. The aim of the present article is to identify the most essential root causes of software defect. The e-service “Invoice Submission” of Riga City Municipality is used as an example. The results of the study can provide useful information for developing improvement activities for e-service higher quality. The analysis is based on the information that is available in the developer’s user request database. The Ishikawa method is used to analyse the causes of defects.