Also experiencing the same problem.
However, in my case,
First, I incorrectly shared a directory as read-only.
I deleted the remote sync,
re-shared as read-write, with the same destination directory
only NOW did this problem arise.
I think that it's an internal resilio sync thing. Resilo sync is not doing some re-check of permissions, (or doing the check and ignoring / not storing the results) and then giving an incorrect error.
Resilio Sync 2.7.2 (1375)
Linux 4.19.0-4-amd64 #1 SMP Debian 4.19.28-2 (2019-03-15) x86_64 GNU/Linux