Inclusion Criteria:

• Experiencing persistent fatigue with a significant impact on daily functioning (Chalder Fatigue- Scale bimodal ≥ 4)

Exclusion Criteria:

• Have insufficient proficiency in German to complete study-related questionnaires
• Lack the cognitive capacity to understand study procedures (e.g. due to dementia)
• Present with severe medical conditions that may interfere with participation (e.g., advanced organ failure, uncontrolled inflammatory or autoimmune diseases, or severe psychiatric disorders)
• Have active cancer or are currently undergoing cancer treatment
• Are deemed incompatible with group participation during the preparation phase, based on a team assessment. Reasons for exclusion based on group incompatibility include:

(-Need for a level of individual therapeutic support that exceeds what can be provided in a group setting; -Refusal to engage in group-based therapy; -Personality traits that are considered incompatible with group dynamics; -Failure to attend the mandatory preparatory webinar (no-show is considered a no-go); -Other significant reasons as determined by the therapeutic team)