Installed Floccus and registered an account in OwnDrive to backup bookmarks. The addon never synced.

At the left of the "Options" and "Sync Now" buttons it says "X Error!" and immediately below the buttons an error description says "JSON.parse: unexpected character at line 1 column 1 of the JSON data".
No idea what to do, error is not self-explanatory and does not give any hints regarding which JSON is crippled, nor a dump of such file; checked the GitHub site but could not make heads nor tails of the code (not a JS dev, more of a C# and VB.Net one I am).

Checked account "https://my.owndrive.com/" with my username and password and it browses OK. Would like the author to try an OwnDrive account, as they say it supports NextCloud, and post his findings, or at least a basic tutorial on how to set up Floccus with a third-party server, not just with localhost. I am not really sure of the URL to use; tried several (taken from the site being browsed in Firefox) to no avail. I'm sure I'm the one doing something wrong, but there are no good working examples of Floccus setups in the Mozilla not Addon Home pages.

Using FF 59.0.2 (64-bit) on Ubuntu 16.04.

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EDIT 1: Cannot reply to your reply, so using this edit.
Thanks a lot for the response.
No, I did not enable or install a Bookmarks app, and my guess is that I would have to do that on OwnDrive.
OwnDrive has its own applet that one would drag to FF Bookmarks to upload, but I guess that is not the one you refer to. Also, no versions given.
Can you help me providing an adequate Bookmarks App link? If it has install instructions for third-party servers like OwnDrive, much better.

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EDIT 2: Cannot reply to your reply, so using this edit.
Also tried Wolkli, does not even have a Bookmarks secction, looks like they removed it.
None of OwnDrive or Wolkli allow adding apps to their free accounts. So the addon isn't any good to me. It could work for people that have their own NextCloud server, but not my case. Too bad.
In view of this, I'll give 4 stars to the addon.