Distributed computing infrastructures are commonly used through scientific gateways, but operating these gateways requires important human intervention to handle operational incidents. This paper presents a self-healing process that quantifies incident degrees of workflow activities from metrics measuring long-tail effect, application efficiency, data transfer issues, and site-specific problems. These metrics are simple enough to be computed online and they make little assumptions on the application or resource characteristics. Incidents are classified in levels and associated to sets of healing actions that are selected based on association rules modeling correlations between incident levels. The healing process is parametrized on real application traces acquired in production on the European Grid Infrastructure. Implementation and experimental results obtained in the Virtual Imaging Platform show that the proposed method speeds up execution up to a factor of 4 and properly detects unrecoverable errors.