Fresh setup with 2 devices, sync stuck with no progress

Support
8

Device A outputs:

curl -X GET -H "X-API-Key: XXX" "http://localhost:20080/rest/db/file?folder=wm9iy-ye9fd&file=lanprov/Spiele/RedAlert1/Maps/Snapshot"
{
  "availability": null,
  "global": {
    "deleted": false,
    "ignored": false,
    "invalid": false,
    "localFlags": 0,
    "modified": "2017-07-29T12:30:09.7972859+02:00",
    "modifiedBy": "V4GPC5C",
    "mustRescan": false,
    "name": "lanprov/Spiele/RedAlert1/Maps/Snapshot",
    "noPermissions": false,
    "numBlocks": 0,
    "permissions": "0777",
    "sequence": 4034,
    "size": 128,
    "type": "DIRECTORY",
    "version": [
      "V4GPC5C:1"
    ]
  },
  "local": {
    "deleted": false,
    "ignored": false,
    "invalid": false,
    "localFlags": 0,
    "modified": "2017-07-29T12:30:09.7972859+02:00",
    "modifiedBy": "V4GPC5C",
    "mustRescan": false,
    "name": "lanprov/Spiele/RedAlert1/Maps/Snapshot",
    "noPermissions": false,
    "numBlocks": 0,
    "permissions": "0777",
    "sequence": 4034,
    "size": 128,
    "type": "DIRECTORY",
    "version": [
      "V4GPC5C:1"
    ]
  }
}