I just found that Firefox shows only folders in my toolbar not RSS feed. Also I found in online database same failure. Now there is with same name RSS and folder. Even I remove folder from online, I can't get RSS feed.
The same problem with my Firefox: each rss feed is converted to an empty folder at uploading to Xmarks server. I'm running FF 56.0 under Linux Mint 18, Mint 17 and FF 52 ESR 32 bit under W10. Xmarks seems not to be able to handle dynamic links/bookmarks anymore. Problem started between Oct 29 and Nov 1. Maybe I'll use Xmarks again, after there will be a solution.
Having the same problem, suddenly can't sync RSS. It used to work and now only sends an empty folder to the server for each RSS feed. There doesn't seem to be anyone at Xmarks acknowledging this problem.
Same problem for me. I had to disable XMarks for the last few days due to the problem with getting dozens of login windows. I re-enabled it yesterday and was able to log in again. This morning it told me there was a problem and I could see that many bookmarks were missing. I rolled back to a previous backup only to find that every RSS feed is now just an empty folder (and I have dozens of these).
I've gone back as far back as September to try to restore them but they're all the same so it's looking very much like LastPass has completely broken the whole lot.