I did backup onto the server. It worked fine! I created a new server backup. (Although 
seemingly "Restore" didn't anything. What should do the "Restore" button?)

EDIT: I can see: The "Restore" button rebuild the folders and items on the server (in the EverHelper), where I can download those again from.

Concl: Server backup and restore work fine for SD and Bookmarks! 

I found the previously mentioned "local backup" on an other browser, that had the Eversync extension, too. 

This means, that the "Local backups" exist in the browser instances? 

Oops! I've found an "accidental" Local Backups from august. So I could rebuild my Speed Dial almost perfectly.  :D

For me now works the Upload and Download correctly.

This asking was for SpudGM?

I've tried. If I got an unsuccessful upload, it deletes the server items, too.

And the next auto synchronizing deletes the items in the browser. (I suppose)

I use ESET Endpoint Antivirus

I've lost my all Speed Dial today. Bookmarks are correct, but I haven't any SP item in EverSync.:(

I can see, that this problem didn't touch me only. :(

I couldn't upload my Speed Dials to Eversync. I suppose, that this unsuccesful uploading deleted the SP items on the server.

(I use Vivaldi with FVD Speed Dial and Eversync.I reinstalled Eversync. I could upload some times the reinserted speed dials, but after some uploads received this popup, and in Everhelper.me disappeared all my SD.)