Problems migrating mattermost to 6.1

I've upgraded an ombibus CE container running a standalone mattermost from version 14.5.0 to 14.6.0

I see the following errors in the logs:

==> /var/log/gitlab/mattermost/mattermost.log <==
{"timestamp":"2021-12-22 17:26:47.923 +01:00","level":"fatal","msg":"Failed to alter column type. It is likely you have invalid JSON values in the column. Please fix the values manually and run the migration again.","caller":"sqlstore/store.go:906","error":"pq: default for column \"timezone\" cannot be cast automatically to type jsonb","tableName":"Users","columnName":"Timezone"}

==> /var/log/gitlab/mattermost/current <==
2021-12-22_16:26:47.92406 {"timestamp":"2021-12-22 17:26:47.923 +01:00","level":"fatal","msg":"Failed to alter column type. It is likely you have invalid JSON values in the column. Please fix the values manually and run the migration again.","caller":"sqlstore/store.go:906","error":"pq: default for column \"timezone\" cannot be cast automatically to type jsonb","tableName":"Users","columnName":"Timezone"}

==> /var/log/gitlab/postgresql/current <==
2021-12-22_16:26:47.89875 ERROR:  column "parentid" does not exist at character 68
2021-12-22_16:26:47.89876 STATEMENT:  UPDATE Posts SET RootId = ParentId WHERE RootId = '' AND RootId != ParentId
2021-12-22_16:26:47.92374 ERROR:  default for column "timezone" cannot be cast automatically to type jsonb
2021-12-22_16:26:47.92375 STATEMENT:  ALTER TABLE users ALTER COLUMN timezone TYPE jsonb USING timezone::jsonb

==> /var/log/gitlab/mattermost/mattermost.log <==
{"timestamp":"2021-12-22 17:26:49.608 +01:00","level":"fatal","msg":"Failed to alter column type. It is likely you have invalid JSON values in the column. Please fix the values manually and run the migration again.","caller":"sqlstore/store.go:906","error":"pq: default for column \"timezone\" cannot be cast automatically to type jsonb","tableName":"Users","columnName":"Timezone"}

==> /var/log/gitlab/mattermost/current <==
2021-12-22_16:26:49.44620 {"timestamp":"2021-12-22 17:26:49.445 +01:00","level":"info","msg":"Server is initializing...","caller":"app/server.go:267","go_version":"go1.16.7"}
2021-12-22_16:26:49.44622 {"timestamp":"2021-12-22 17:26:49.445 +01:00","level":"info","msg":"Starting websocket hubs","caller":"app/web_hub.go:93","number_of_hubs":96}
2021-12-22_16:26:49.45948 {"timestamp":"2021-12-22 17:26:49.459 +01:00","level":"info","msg":"Loaded system translations","caller":"i18n/i18n.go:93","for locale":"en","from locale":"/opt/gitlab/embedded/service/mattermost/i18n/en.json"}
2021-12-22_16:26:49.47049 {"timestamp":"2021-12-22 17:26:49.470 +01:00","level":"info","msg":"Pinging SQL","caller":"sqlstore/store.go:261","database":"master"}
2021-12-22_16:26:49.47559 {"timestamp":"2021-12-22 17:26:49.475 +01:00","level":"info","msg":"Pinging SQL","caller":"sqlstore/store.go:261","database":"replica-0"}
2021-12-22_16:26:49.47934 {"timestamp":"2021-12-22 17:26:49.479 +01:00","level":"info","msg":"Pinging SQL","caller":"sqlstore/store.go:261","database":"migrations"}
2021-12-22_16:26:49.60873 {"timestamp":"2021-12-22 17:26:49.608 +01:00","level":"fatal","msg":"Failed to alter column type. It is likely you have invalid JSON values in the column. Please fix the values manually and run the migration again.","caller":"sqlstore/store.go:906","error":"pq: default for column \"timezone\" cannot be cast automatically to type jsonb","tableName":"Users","columnName":"Timezone"}

==> /var/log/gitlab/postgresql/current <==
2021-12-22_16:26:49.54360 ERROR:  relation "idx_posts_channel_id" does not exist
2021-12-22_16:26:49.54363 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54373 ERROR:  relation "idx_channels_name" does not exist
2021-12-22_16:26:49.54375 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54387 ERROR:  relation "idx_publicchannels_name" does not exist
2021-12-22_16:26:49.54388 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54404 ERROR:  relation "idx_channelmembers_channel_id" does not exist
2021-12-22_16:26:49.54405 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54420 ERROR:  relation "idx_emoji_name" does not exist
2021-12-22_16:26:49.54421 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54433 ERROR:  relation "idx_oauthaccessdata_client_id" does not exist
2021-12-22_16:26:49.54434 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54448 ERROR:  relation "idx_oauthauthdata_client_id" does not exist
2021-12-22_16:26:49.54449 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54461 ERROR:  relation "idx_preferences_user_id" does not exist
2021-12-22_16:26:49.54461 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54477 ERROR:  relation "idx_notice_views_user_id" does not exist
2021-12-22_16:26:49.54478 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54491 ERROR:  relation "idx_notice_views_user_notice" does not exist
2021-12-22_16:26:49.54492 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54511 ERROR:  relation "idx_status_user_id" does not exist
2021-12-22_16:26:49.54511 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54525 ERROR:  relation "idx_teammembers_team_id" does not exist
2021-12-22_16:26:49.54525 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54538 ERROR:  relation "idx_teams_name" does not exist
2021-12-22_16:26:49.54540 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54556 ERROR:  relation "idx_user_access_tokens_token" does not exist
2021-12-22_16:26:49.54557 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54572 ERROR:  relation "idx_user_terms_of_service_user_id" does not exist
2021-12-22_16:26:49.54573 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54586 ERROR:  relation "idx_users_email" does not exist
2021-12-22_16:26:49.54586 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54603 ERROR:  relation "idx_sharedchannelusers_user_id" does not exist
2021-12-22_16:26:49.54604 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.54616 ERROR:  relation "idx_retentionpolicies_displayname_id" does not exist
2021-12-22_16:26:49.54617 STATEMENT:  SELECT $1::regclass
2021-12-22_16:26:49.58341 ERROR:  column "parentid" does not exist at character 68
2021-12-22_16:26:49.58342 STATEMENT:  UPDATE Posts SET RootId = ParentId WHERE RootId = '' AND RootId != ParentId
2021-12-22_16:26:49.60847 ERROR:  default for column "timezone" cannot be cast automatically to type jsonb
2021-12-22_16:26:49.60849 STATEMENT:  ALTER TABLE users ALTER COLUMN timezone TYPE jsonb USING timezone::jsonb

These errors appear in what appears to be an infinite loop, and unsurprisingly the server doesn't start 😞

Let me know if there's any additional information I can provide.