Our understanding of the underlying psychological processes of development, maintenance, and treatments for stress-induced exhaustion disorder (ED) remains limited. Therefore, the current study aimed to explore whether sleep concerns, pathological worry, perfectionistic concerns, and psychological flexibility mediate change in exhaustion symptoms during a Multimodal intervention for ED based on Cognitive behavioral therapy principles. Participants (N = 913) were assessed at three time points, and mediation was explored using a two-criteria analytical model with linear mixed-effects models (criterion one) and random intercepts cross-lagged panel modeling (criterion 2). Criterion one for mediation was successfully met, as the findings indicated significant associations between time in treatment, with all suggested mediators, and exhaustion symptoms (significant ab-products). However, criterion two was not satisfied as changes in the mediators did not precede changes in exhaustion symptoms. Therefore, mediation could not be established. Instead, changes in the suggested mediators appeared to result from changes in exhaustion symptoms. Consequently, sleep concerns, pathological worry, perfectionistic concerns, and psychological flexibility appear to improve in conjunction with exhaustion symptoms during treatment, where improvement in exhaustion is indicated as the main driving factor, based on this exploratory analysis. The implications of these findings are contextualized within a broader framework of process-based therapy.