MULTI BACKEND : startswith first arg must be bytes or a tuple of bytes, not str

Duplicity 0.8.0.8 (official release manually built)
python -V : Python 3.7.5 (manually built)
Kernel: 4.19.0-0.bpo.6-amd64
Desktop: Openbox 3.6.1
Distro: Devuan GNU/Linux 2.1 (ascii) x86_64

I use the MULTI backend to use "cold" "OVH Public Cloud Archive", following this guide: https://docs.ovh.com/gb/en/storage/pca/duplicity/#multibackend-setup

I setup a test "hot" container (legacy Swift Openstack backend) named "thierrybo-desk_tests_hot" and a "cold" container using PCA backend named "thierrybo-desk_tests_cold"

My first run :

duplicity --verbosity debug --asynchronous-upload --file-prefix-manifest 'hot_' --file-prefix-signature 'hot_' --file-prefix-archive 'cold_' ~/PDF/ "multi://$HOME/.config/duplicity/config_v3_ovh_pca-thierrybo-desk_tests.json?mode=mirror&onfail=abort"

the key error message (see the attached file for full log):

  File "/home/thierrybo/.local/lib/python3.7/site-packages/duplicity/backends/multibackend.py", line 220, in <listcomp>
    matching_prefixes = [k for k in list(self.__affinities.keys()) if filename.startswith(k)]
 TypeError: startswith first arg must be bytes or a tuple of bytes, not str

Here is my "config_v3_ovh_pca-thierrybo-desk_tests.json" file (obfuscated) :

[
    {
        "description": "Cold storage",
        "url": "pca://thierrybo-desk_tests",
        "env": [
            {
                "name": "PCA_AUTHURL",
                "value": "https://auth.cloud.ovh.net/v3"
            },
             {
                 "name": "PCA_AUTHVERSION",
                 "value": "3"
             },
             {
                 "name": "PCA_PROJECT_DOMAIN_NAME",
                 "value": "Default"
             },
             {
                 "name": "PCA_USER_DOMAIN_NAME",
                 "value": "Default"
             },
             {
                 "name": "PCA_TENANTID",
                 "value": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
             },
             {
                 "name": "PCA_PROJECTID",
                 "value": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
             },
             {
                 "name": "PCA_USERNAME",
                 "value": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
             },
             {
                 "name": "PCA_PASSWORD",
                 "value": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
             },
             {
                 "name": "PCA_REGIONNAME",
                 "value": "GRA"
             }
         ],
         "prefixes": ["cold_"]
     },
     {
         "description": "Hot storage",
         "url": "swift://thierrybo-desk_tests_hot",
         "env": [
             {
                 "name": "SWIFT_AUTHURL",
                 "value": "https://auth.cloud.ovh.net/v3"
             },
             {
                 "name": "SWIFT_AUTHVERSION",
                 "value": "3"
             },
             {
                 "name": "SWIFT_PROJECT_DOMAIN_NAME",
                 "value": "Default"
             },
             {
                 "name": "SWIFT_USER_DOMAIN_NAME",
                 "value": "Default"
             },
             {
                 "name": "SWIFT_TENANTID",
                 "value": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
             },
             {
                 "name": "SWIFT_PROJECTID",
                 "value": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
             },
             {
                 "name": "SWIFT_USERNAME",
                 "value": "XXXXXXXXXXXX"
             },
             {
                 "name": "SWIFT_PASSWORD",
                 "value": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
             },
             {
                 "name": "SWIFT_REGIONNAME",
                 "value": "GRA"
             }
         ],
         "prefixes": ["hot_"]
     }
 ]