A Study to Find a Suitable Dose of BI 1821736 and Test Whether it Helps People With Advanced Cancer

Inclusion Criteria: * Histologically confirmed diagnosis of malignant tumor. * Advanced, unresectable and/or metastatic or relapsed/refractory solid tumors. * Has failed conventional treatment or for whom no therapy of proven efficacy exists or who is not eligible for established treatment options. Patient must have exhausted available treatment options known to prolong survival for their disease. * Has at least one tumoral lesion which is amenable to biopsy. * Signed and dated, written informed consent form (ICF) in accordance with ICH-GCP and local legislation obtained prior to any trial-specific procedures, sampling, or analyses that are not part of normal standard of practice care. * Eastern Cooperative Oncology Group score of 0 or 1. * Adequate organ function or bone marrow reserve defined as demonstrated at screening by the following laboratory values: * Absolute neutrophil count (ANC) ≥ 1.5 x 109/L (≥ 1.5 x 10\^3/μL)(≥ 1500/mm\^3); haemoglobin ≥ 90 g/L (≥ 9.0 g/dL)(≥ 5.6 mmol/L); platelets ≥ 100 x 10\^9/L (≥ 100 x 10\^3/μL)(≥ 100 x 10\^3/mm\^3) without the use of haematopoietic growth factors within 4 weeks of start of trial medication. * Creatinine ≤ 1.5 times the upper limit of normal (ULN). * Total bilirubin ≤ 1.5 times the ULN, except for patients with Gilbert's syndrome: total bilirubin ≤ 3 times ULN or direct bilirubin ≤ 1.5 times ULN. * Aspartate transaminase (AST) and alanine transaminase (ALT) ≤ 3 times ULN if no demonstrable liver metastases, or otherwise ≤ 5 x ULN if transaminase elevation is attributable to liver metastases. * Partial thromboplastin (PT) / activated partial thromboplastin time (aPTT) \<1.5 times ULN unless on a stable dose of an anticoagulant. * All toxicities related to previous anti-cancer therapies have resolved to ≤ Common Terminology Criteria for Adverse Events (CTCAE) Grade 1 prior to trial treatment administration (except for alopecia and peripheral neuropathy which must be ≤ CTCAE Grade 2 and amenorrhea/menstrual disorders which can be any grade). * Further inclusion criteria apply Exclusion Criteria: * Major surgery (major according to the investigator's assessment) performed within 4 weeks prior to start of study treatment. * Previous treatment with Vesicular Stomatitis Virus (VSV)-based agents. * Patients with brain metastases unless they have completed brain radiotherapy and are asymptomatic. * Radiotherapy within 4 weeks prior to the start of study treatment, except in case of a brief course of palliative radiotherapy (e.g. for analgesic purpose or for lytic lesions at risk of fracture) which can then be completed within two weeks prior to start of study treatment. Note: No radiation must have been given to any lesions planned to be biopsied within 6 months of start of treatment. * Prior (within 3 weeks of first dose) or concomitant use of systemic corticosteroids (\>10 mg daily prednisone or equivalent). * Prior (within 3 weeks of first dose or less than 5 half-lives) or concomitant use of a medication or a condition considered a high risk for complications from biopsy as per the Investigator's judgement. * Prior (within 3 weeks of first dose or less than 5 half-lives) or concomitant use of interferon, immunotherapy agents, or tamoxifen. * Active infection requiring systemic therapy (antibacterial, antiviral, antiparasitic or antifungal therapy) at the start of treatment in the trial. * Further exclusion criteria apply