There will be a total of four assessments (see primary & secondary endpoints). The first visit of the study will be a screening and baseline (T0) visit. After the baseline assessment, the subjects will be randomized to either of the two treatment arms. Within one week, the first of the successive bi-weekly 50-minute individual face-to-face treatment sessions will be performed by a study therapist. After 7 therapy sessions, the primary and secondary outcomes will be obtained in week 5 (T1) by the blinded rater. This will be followed by the treatment sessions 8 to 15 which will be carried out by the therapist. End-of-treatment primary and secondary outcomes will be obtained in week 10 (T2). The final follow-up assessment (T3) will be performed 6 months after randomization by the blinded rater.

Underlying mechanisms are examined using neuroimaging. MRI data are acquired at four scanning sites and will be performed at baseline, end-of-treatment and follow-up to obtain a high-resolution structural T1-weighted image, a T2-weighted FLAIR image, a resting state fMRI, and diffusion tensor imaging (DTI) of the subjects' brain.

Blood sampling will be acquired in order to investigate the underlying mechanisms in LLD and the specific effects of psychotherapy. Blood samples are acquired at five sites at baseline (T0), T1, T2 and T3 for genetic and epigenetic analyses, measurement of Amyloid-β, Neurofilament light chain (NFL), Peripheral Blood Mononuclear Cells (PBMCs), Metabolomics, Proteomics and miRNA analyses.

A secondary outcome measure is the change in depression severity from baseline to month 6 measured by the 30-item Geriatric Depression Scale (GDS). It is a self-rating scale including 30 Items in a yes/no format and a total score range from 0 to 30. Higher values represent worse outcome.

A secondary outcome measure is the change in depression severity from baseline to week 5 measured by the 16-item Quick Inventory of Depressive Symptomatology Score - Clinician Rating (QIDS-C). It is a clinician-rated scale including 16 Items and a total score range from 0 to 27. Higher values represent worse outcome.

A secondary outcome measure is the change in depression severity from baseline to week 10 measured by the 16-item Quick Inventory of Depressive Symptomatology Score - Clinician Rating (QIDS-C). It is a clinician-rated scale including 16 Items and a total score range from 0 to 27. Higher values represent worse outcome.

A secondary outcome measure is the change in depression severity from baseline to month 6 measured by the 16-item Quick Inventory of Depressive Symptomatology Score - Clinician Rating (QIDS-C). It is a clinician-rated scale including 16 Items and a total score range from 0 to 27. Higher values represent worse outcome.

A secondary outcome measure is the change in depression severity from baseline to week 10 measured by the 35-item Patient Reported Outcome for Major Depressive Disorder (PRO-MDD) Score. It is a self-rating scale including 35 Items on an11-point numeric rating scale. Higher values represent worse outcome.

A secondary outcome measure is the change in depression severity from baseline to month 6 measured by the 35-item Patient Reported Outcome for Major Depressive Disorder (PRO-MDD) Score. It is a self-rating scale including 35 Items on an11-point numeric rating scale. Higher values represent worse outcome.

A secondary outcome measure is the longitudinal evaluation of depressive symptoms by the Longitudinal Interval Follow-up Evaluation Interview (LIFE) at month 6.

A secondary outcome measure is the change in insomnia severity from baseline to week 10 measured by the 7-item Insomnia Severity Index (ISI). It is a self-rating scale including 7 Items rated on a scale from 0 to 4 from less to more severe. The total score ranges from 0 to 28. Higher values represent worse outcome.

A secondary outcome measure is the change in insomnia severity from baseline to month 6 measured by the 7-item Insomnia Severity Index (ISI). It is a self-rating scale including 7 Items rated on a scale from 0 to 4 from less to more severe. The total score ranges from 0 to 28. Higher values represent worse outcome.

A secondary outcome measure is the change in sleepiness from baseline to week 10 measured by the 8-item Epworth Sleepiness Scale (ESS). It is a self-rating scale including 8 Items rated on a 4-point Likert scale. The total score ranges from 0 to 24. Higher values represent worse outcome.

A secondary outcome measure is the change in sleepiness from baseline to month 6 measured by the 8-item Epworth Sleepiness Scale (ESS). It is a self-rating scale including 8 Items rated on a 4-point Likert scale. The total score ranges from 0 to 24. Higher values represent worse outcome.

A secondary outcome measure is the change in REM sleep behavior from baseline to week 10 measured by the 10-item REM Sleep Behavior Disorder Screening Questionnaire (RBDSQ). It is a self-rating scale including 10 Items in a yes/no format. The total score ranges from 0 to 13. Higher values represent worse outcome.

A secondary outcome measure is the change in REM sleep behavior from baseline to month 6 measured by the 10-item REM Sleep Behavior Disorder Screening Questionnaire (RBDSQ). It is a self-rating scale including 10 Items in a yes/no format. The total score ranges from 0 to 13. Higher values represent worse outcome.

A secondary outcome measure is the change in anxiety symptoms from baseline to week 10 measured by the 20-item Geriatric Anxiety Inventory (GAI). It is a self-rating scale including 20 Items in a agree/disagree format and a total score range from 0 to 20. The cut-off score of 9 classifies the presence of clinically significant anxiety. Higher values represent worse outcome.

A secondary outcome measure is the change in anxiety symptoms from baseline to month 6 measured by the 20-item Geriatric Anxiety Inventory (GAI). It is a self-rating scale including 20 Items in a agree/disagree format and a total score range from 0 to 20. The cut-off score of 9 classifies the presence of clinically significant anxiety. Higher values represent worse outcome.

A secondary outcome measure is the change in quality of life from baseline to week 10 measured by the 24-item WHOQOL-OLD and 26-item WHOQOL-BREF.

A secondary outcome measure is the change in quality of life from baseline to month 6 measured by the 24-item WHOQOL-OLD and 26-item WHOQOL-BREF.

A secondary outcome measure is the change in overall health status from baseline to week 10 measured by the 36-item Short Form Health Survey (SF-36).

A secondary outcome measure is the change in overall health status from baseline to month 6 measured by the 36-item Short Form Health Survey (SF-36).

A secondary outcome measure is the change in subjective cognitive functioning from baseline to month 6 measured by the semi-structured Subjective Cognitive Decline interview.

A secondary outcome measure is the change in cognitive function from baseline to month 6 measured by the Consortium to Establish a Registry for Alzheimer's Disease (CERAD-Plus) neuropsychological test battery.

A secondary outcome measure is the change in executive function from baseline to month 6 measured by the Neuropsychological Assessment Battery (NAB) maze subtest.

Assessment of the severity of five categories of childhood trauma(emotional/physical/sexual abuse and emotional/physical neglect) at baseline

10-item scale measuring the personality traits Extraversion, Agreeableness, Conscientiousness, Emotional Stability, and Openness to assess the influence of personality traits on treatment outcome at baseline. The items are rated on a five-step scale from 1 "disagree strongly" to 5 "agree strongly". The scale consists of 2 BFI items for each Big Five Dimension.