Sync error and password not saved

Hallo.
This problem popped up a couple of version ago, when Firefox switched from Option panel in floating window to in-browser page.
I can't say if the two things are connected, but I suspect.

First, I can connect to my Firefox account, but I receive a "Sync encountered an error while syncing. Unknown error. Sync will automatically retry this action" message at the bottom of the window, and neither automatically nor manually the syncing operation succeed.

furthermore, even if the "Remember password for sites" in the Options/Security panel is checked, it simply don't and the "Stored password" box is empty.

thanks for any help or suggestions.