Inclusion Criteria:

          -  Adult males or females aged 18 years or over

          -  Disease status: - Diagnosis of metastatic or advanced CRC, UCC and SCCHN not
             responding to standard therapy or for whom no standard treatment exists

          -  Prior palliative radiotherapy completed at least 3 weeks before study treatment
             administration

          -  For patients who have received prior PD-1 / PD-L1 therapy (Cohorts 7A and 7B and dose
             expansion phase only): Prior PD-1 / PD-L1 inhibitor therapy in current line of
             treatment for ≥6 weeks and ≤4 months, with best response of stable disease or
             progressive disease

          -  ECOG performance status 0 or 1

          -  Predicted life expectancy of 3 months or more

          -  Ability to comply with study procedures in the Investigator's opinion

          -  Recovered to Grade 1 from the effects (excluding alopecia) of any prior therapy for
             their malignancies Adequate lung function

          -  Adequate renal function

          -  Adequate hepatic function

          -  Adequate bone marrow function:

          -  Adequate coagulation tests: international normalized ratio (INR) ≤1.5

          -  Meeting reproductive status requirements

          -  Subjects must provide written informed consent to participate

          -  Willing to consent to tumour biopsies during the study

        Exclusion Criteria:

          -  Pregnant or breastfeeding females

          -  Known history or evidence of significant immunodeficiency due to underlying illness

          -  Splenectomy

          -  Prior allogeneic or autologous bone marrow or organ transplantation

          -  Any history of renal disease, renal injury or auto-immune disease.

          -  History of idiopathic pulmonary fibrosis, drug induced pneumonitis, evidence of active
             pneumonia or pneumonitis on computed tomography scan

          -  Clinically or radiologically suspected, or evidence of, lymphangitic carcinomatosis

          -  Active infections requiring antibiotics, physician monitoring or recurrent fevers
             >100.4˚F (38.0˚C) associated with a clinical diagnosis of active infection

          -  Active viral disease or positive test for hepatitis B virus using hepatitis B surface
             antigen test or positive test for hepatitis C virus (HCV) using HCV ribonucleic acid
             or HCV antibody test indicating acute or chronic infection. Positive test for HIV or
             AIDS; testing is not required in the absence of history

          -  Use of the following antiviral agents: ribavirin, adefovir, lamivudine or cidofovir
             within 7 days prior to the first dose of study treatment; or pegylated interferon in
             the 4 weeks before the first dose of study treatment

          -  For Cohort 1 to 6:Prior treatment with PD-1 and programmed death ligand (PD-L)1
             inhibitors

          -  For Cohorts 7A and 7B and dose expansion phase: History of Grade >2 or currently
             uncontrolled immune-related AEs while being treated with PD-1 / PD-L1 inhibitors

          -  For Cohorts 7A and 7B and dose expansion phase: Patients with progressive disease with
             ≥50% increase in disease burden from start of PD-1 / PD-L1 inhibitor therapy in the
             most recent line of treatment are excluded

          -  Major surgery or treatment with any chemotherapy (bisphosphonate therapy or treatment
             with receptor activator of nuclear factor kappa-Β l(RANK)-ligand inhibitors for
             metastatic bone disease is permitted), biologics for cancer or investigational therapy
             in the 28 days before the first dose of study treatment (patients with prior cytotoxic
             or investigational products <3 weeks prior to study treatment might be eligible after
             discussion between the Investigator and Medical Monitor, if toxicities from the prior
             treatment have been resolved to NCI CTCAE Grade 1 and decision is supported by the
             half-life of previous therapy). All toxicities attributed to prior anti-cancer therapy
             other than alopecia must have resolved to Grade 1 or baseline before the first dose of
             study treatment. Patients with toxicities attributed to prior anti-cancer therapy
             which are not expected to resolve and result in long lasting sequelae, such as
             neuropathy after platinum based therapy, are permitted to enroll Note: This does not
             apply to anti-PD-1 / PD-L1 if in the patient's current line of therapy, and anti-PD-1
             / PD-L1 treatment may continue during screening

          -  Other prior malignancy active within the previous 3 years except for local or organ
             confined early stage cancer that has been definitively treated with curative intent,
             does not require ongoing treatment, has no evidence of residual disease and has a
             negligible risk of recurrence and is therefore unlikely to interfere with the primary
             and secondary endpoints of the study, including response rate and safety

          -  Symptomatic brain metastases or any leptomeningeal metastasis that is symptomatic
             and/or requires treatment. Patients with brain metastases are eligible if these have
             been locally treated (surgery, radiotherapy). There must also be no requirement for
             immunosuppressive doses of systemic corticosteroids (>10 mg/day prednisone equivalent)
             for at least 14 days before the first dose of study treatment. Both surgery and or
             radiotherapy must have been completed at least 2 weeks before first dose of study
             treatment

          -  Any serious or uncontrolled medical disorder that, in the opinion of the Investigator
             or the Medical Monitor, may increase the risk associated with study participation or
             study treatment administration, impair the ability of the patient to receive protocol
             therapy or interfere with the interpretation of study results

          -  Known allergy to enadenotucirev, nivolumab or their excipients

          -  Any other medical or psychological condition that would preclude participation in the
             study or compromise ability to give informed consent

          -  Dependence on continuous supplemental oxygen use

          -  History of myocardial infarction or significant cardiovascular or cerebrovascular
             event in the 12 months before the first dose of study treatment

          -  An increased risk due to tumour flare, as assessed by the Investigator (e.g. an
             initial increase in tumour size that may lead to intestinal obstruction, obstruction
             of the ureter or airway)

          -  Penetrating tumour infiltration of major blood vessels, pericardium, gastrointestinal
             tract or other hollow organs that may lead to perforation due to tumour necrosis

          -  History of DVT or pulmonary embolus in the 12 months before the first dose of study
             treatment

          -  History of significant bleeding requiring hospitalization in the 12 months before the
             first dose of study treatment

          -  Patients receiving therapeutic or prophylactic anticoagulation therapy